mon/MonClient: one-shot mon connection on started to fetch config

mon/MonClient: one-shot mon connection on started to fetch config

This is not particularly efficient, but it works:

 - connect to the monitor to get the monmap and config
 - tear it all down
 - proceed with normal startup (which presumably involves reconnecting
   to the mon all over again).

This allows us to set config options that may affect the mon communication
itself, like ms_type.

Signed-off-by: Sage Weil <sage@redhat.com>