* Resolve java lint problem with soft link of directory.

* Ignore the soft link of directory.

* Add timestamp

* fix vid

* Address dutor's comment

* Address CPWstatic's comment and Add type cast handle in TraverseExecutor

* update

* update feroda deps

* implement delete a vertex

* address the comments of wadeliuyi and dangleptr

* address dangleptr's comments

* address comments of dangleptr and critical27

* address darionyaphet's comments

* Added support to singular forms for STEPS and VALUES

* Make VALUES unreserved

* enhance java client

* fix docs/nGQL-tutorial.md

Basic leader balance for storage, keep trying to trans leader from host with most leader to host with least leader, until all balanced.

Modify/Add two command in console:

1. "show host" will also display total leader count and leader count in each space
2. add command "balance leader", which will try to balance leader between different hosts

* test edge miss

* fix mutli version

* register gflags in meta client

* parse json

* try register at first operation

* address @laura-ding's comments

* fix ut, address comments

* address laura-dings's comments

* address comments

* address @dangleptr's comments, update gflags.json

* address @dangleptr's comments

* Ignore ccls configuration files

* Fix variable syntax document about load_data_interval_secs

* Cleanup unusable load_config_interval_secs configuration option

* Implemented type casting

* Address @laura-ding's comments

* Address comments

* Add build doc

* Address whitewum's comment

* Address yixinglu's comment

* Address whitewum's comment

* Modify SKIP_JAVA_CLIENT default is ON

* Add java jdk

* Modify spelling

* Modify resolve

* Add space size instructions

* add create-tag-edge, show and update get-started

* update

* update

* update

* update

* add FAQ

* update

* update

* update

* update

* file structure

* add create-tag-edge, show and update get-started

* update

* add FAQ

* update

* update

* update

* update

* file structure

* change build

* update

* update

* update

* update

* update

* last update

* update

* update

* update doc

* update

* update

* update

* update

* Issue#655 support for building with Clang

Compiled by clang 7.0.1((Fedora 7.0.1-6.fc29)

* Modified some of the code according to sherman's comments.

* add create-tag-edge, show and update get-started

* update

* update

* update

* update

* add FAQ

* update

* update

* update

* update

* file structure

* add create-tag-edge, show and update get-started

* update

* add FAQ

* update

* update

* update

* update

* file structure

* change build

* update

* update

* update

* update

* update

* last update

* update

* update

* add log

* update

* change graph

* change graph

* update log

* update log

* update log

* upload log

* update log

* update badge

* add create space syntax

* update link

* update

* improve unreserved keywords feature

support time to live

alter tag/edge drop column, and the column to be delete is as TTL column

address dangleptr's comment

* address laura-ding's comments

* rebase master

* Fixed crash problem due to meta not ready

* add comments

* Refactor clusterId and ActiveHostMan, change FLAGS_hosts_whitelist_enabled to be false by default

* Address all comments

This time only temporarily avoided this crash,
In issue #192, I will solve this problem for better.

* cluster depoly doc

* fix sherman-the-tank's comment

* fix graph port

* Enhance Java Importer

* fix ranking type

* update copyright 2019

* add README.md

*     Issue#424 to make `Expression::eval` to be able to do error handling.

    Summary:

    1. When the expression is evaluated, some wrong judgments are added,
       ifthe expression was failed, an error will be replyed to the user.

    2 fixed the crash when using mockStorageServer.

* 1 reformat the code.
2 Handled some errors.

* Implement union.

* Add distincter.

* Pipe have priority over set operator.

* Ajust error handling.

* Add distinct and implicit type casting for union.

* Inplement intersect and minus.

* Fix format.

* Adrress @monabobo's comment.

* Add result column names.

* Delete distincter and rebase.

* Rebase and fix conflict.

* Address @whitewum's comment.

* Address @laura-ding's comment and rebase.

* Fix crash problem when stop storage/meta daemon

* Address darion's comments

* Address wadeliuyi's comments

* Add fetch prop sentence.

* Add fetch executor.

* Refactoring expression.

* Accomplish fetch tag prop.

* Add ut for fetch vertex.

* Accomplish fetch edge.

* Add ut.

* Refactoring.

* Adjust ut.

* Fix format.wa

* Address @wadeliuyi's comment.

* Fix conflict.

* Refactoring.

* Add distinct support.

* Address comment.

* Fix conflict.

* Fix conflict and adjust distinct.

* Add default value for rank in EdgeRowItem. And collect prop after eval.

* Address laura-ding's comment.

* Fix link error after rebase.

It is used to ingest sst files directly after download files from hdfs. 

We use cluster id to verify whether one host belongs to our cluster or not. 
If the host send heart beat to metad without clusterId,  metad will take it as a new host into our cluster.  Otherwise, metad will check the cluster id received is equals to the local one,  and reject the host request if cluster id conflicts. 

After the cluster id supported,  we don't really need add/remove hosts steps anymore.