Make replication naming conventions consistent

Replication is filled with terrible naming choices, and many inconsistencies. We sometimes use "replset*" and sometimes use "repl_set*", we sometimes use "initialsync*" and sometimes use "initial_sync*".

We also have terrible names like "BackgroundSync", which both is not descriptive and is in a file named "bgsync.cpp" instead of "background_sync.cpp".