* 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.