* Rename storage module to kvstore
* update storage.thrift, refactor QueryResponse
* Add some skeleton class for storage

 * Add ProcessUtils
 * Move getExePath and getExeCWD out from FileUtils to ProcessUtils
 * Support server-relaunch detection through pid file

An rough skeleton of the query engine

Fix compile issues(-isystem) due to PCH

Use strcpy instead of strncpy to suppress the -Wstringop-truncation warning

close #1 

Modified storage.thrift and added meta

Also revised all interface to support GraphSpaceID and PartitionID when communicating with storage service

* Add storage module

* Re-factor the vgraph.thrift

1) Now treat vgraph.thrift as an interface between client and vgraph server, so the data will not be encoded
2) Added storage.thrift as an internal interface between query engine and storage service

* Enhance/fix FileBasedWAL

1) Support log rollback
2) Separate buffer flusher logic, so that the number of flushers is independent from the number of WALs/Shards
3) Improve the multiple threads support, but still not support multiple threads calling appendLogs()/rollbackToLog()
4) Add UT for rollback
5) Fix the flaky UTs

* Rebased and addressed dutor's comments

Turned on `-Wshadow`.

* Enable PCH

* Removed the dependency on engshare

* Fixed common/base dependency

* Fixed build on centos 7.5

* Fixed common/base dependency

* Fixed couple gcc-8.1 warnings

* Added Configuration and TempFile

In addition, Turned on the compiler warnings `-Wall` and `-Werror`.

Resolved some existing warnings, although there are a few `-Wsign-compare` I couldn't figure out.
Supress them with `-Wno-error=sign-compare` for the time being.

Just machanical changes.

It somehow didn't work with the command `cmake -DCMAKE_BUILD_TYPE=Release`.

* First Implementation of GQL parser

* Address @sherman-the-tank 's comments

* Implement syntax of UPDATE/INSERT

In addition, added support for hexadecimal and octal numbers.

* Reserve space on the buffer in `toString`

* Fixes #14 Add more UT for FileBasedWAL

* [Feature] Added some concurrent utilities, GenericThreadPool, etc.

This reverts commit b22b49fbcb6645c085fb1ebf42962b9a37bd1711.