Removing GFlags as a dependency

[kgpai]

Currently we use gflags in flags.cpp . Gflags when used in a library is somewhat problematic since it requires that flags be loaded only once.
However with velox being used in python libraries (like torcharrow) and also for upcoming future libraries (like pyvelox) will need the ability to be loaded multiple times.
For example consider scenario below:

library A -> velox_memory -> flags.cpp . 
library B -> velox_type -> flags.cpp 
The flags defined in flags.cpp, no matter what the names we have in flags, A and B when loaded will crash due to gflags complaining.
so when you do following in python : 
import A
import B // CRASH.

This crash happens due to code here in gflags.

Due to this , I think its best we remove gflags as a dependency. Currently gflags is only used in Memory and Exception tracing. These uses can be refactored out and instead of providing command line options directly we can have an api init (const GlobalOptions&) and have clients take care of parsing them as useful for them. This would be a better approach to have in places where command line options will not be available (like say in python)..