Only access watcher through proxy

This is a fix for powerline-daemon: otherwise the first powerline object that 
did `watcher(file)` will receive file changed event and other powerline objects 
will not.
Remove ThreadedSegment.write_lock

Assuming getattr(self, 'update_value') and setattr(self, 'update_value', value) 
are atomic. True with cpython unless somebody defined __getattribute__ or 
__setattr__.