Replaces .lock()/.unlock() with lock_guard

The _Table_Locker class was holding a mutex pointer and calling lock()
and unlock() through it.  This has been replaced with a std::lock_guard
data member to facilitate the locking and unlocking of the mutex.  This
simplifies the constructor and destructor bodies (now 0 lines each).
Wraps entire synctable.cpp with namespace synclock

For some reason I qualified everything in synctable.cpp with synclock::
instead of just wrapping it in namespace synclock{ }.  Fixed it.