contrib/fb303 does not compile after C++ concurrency code refactor

contrib/fb303 no longer compiles after THRIFT-4730 removed ReadWriteMutex.

The ReadWriteMutex used by fb303 is protecting counter statistics. These are very write-heavy and read-light (these are read only when the Service methods getCounters() or getCounter() are called). Because of this, retaining a read-write mutex to protect these counters is unjustified and we can safely swap this out for a normal Mutex.

Compilation also fails after THRIFT-5168 removed generation of *_constants.cpp and *_constants.h generation because the fb303 Makefile explicitly depends on these unnecessary files