Replace GFlags with GlobalConfig in Velox library

[majetideepak]

Velox currently uses GFlags in the library. Using GFlags is not recommended in a library since these flags have to be declared in the global namespace and can lead to symbol conflicts when the library is linked transitively.

GFlags also adds to the dependency list of Velox and it requires Velox applications to invoke gflags::ParseCommandLineFlags(); which can be avoided.

This change was recently added but reverted here #12503 since it was not backward compatible.

The plan is to add this change back but with backward compatibility support. This is to provide Velox users and applications time to migrate off GFlags to GlobalConfig.
The backwards compatibility support will still keep the dependency between Velox and gflags.

In a month or two, this backwards compatibility support will be removed. Users can then choose to invoke translateFlagsToGlobalConfig() if they want to continue to use GFlags in their application.

[majetideepak]

CC: @kgpai, @assignUser, @kevinwilfong

[assignUser]

Thanks!