master,meta(logger|restore): Do not use .back file

Added class for managing lock files.
Lockfile class is used in master server and metarestore
effectively preventing parallel access to metadata
by those programs.

Master do not rename metadata.mfs file to metadata.mfs.back
anymore. We cleanup and simplify datadir content semantics,
it will allow us to simplify implementation of auto-restart
feature, start/stop consistency is checked with .lock file anyway.

Change-Id: I269d207d4c0ce94905b982bf735a140e21265fa2