SERVER-14668 Move docLockingSupported out of d_concurrency.cpp

It has to be used in numerous places, so it is better suited as a global API.
SERVER-14668 Cleanup MMAPV1-specific classes from db.cpp

o Moves DataFileSync to be under MMAP V1, because it is only used there
o Cleanup variable name collision with replSettings
SERVER-13522 Remove usages of runCount and replace them with DBDirectClient::count
SERVER-14668 Move WriteUnitOfWork around DatabaseCatalogEntry creation outside of DatabaseHolder

This makes it up to the storage engine to decide what transactional
operations to do during database creation.

Cleans up some references to operation context, which were not necessary.