client: refactor mds requests to put most processing in reply handler

client: refactor mds requests to put most processing in reply handler

The goal is to fix race issues between open reply processing (previously
in calling processes context) and subsequent file_cap messages.  This just
pushes generic mds reply handling into the reply handler, including
the ceph_insert_trace (somewhat heavyweight, unfortunately) and the cap
creation/update.

This effectively pushes a lot more code into the single thread thats
doing mds replies, but simplifies a lot of code.  Until scaling issues
arise, I think it's fine.