mon: set next commit in mon command replies

The mon command acks include a version that is used by the client to
determine which version of the map they need to get or wait for in order
to see the effects of their command.  Current we are returning
get_last_committed() everywhere, but we are about to commit something (and
waiting for it), which will increase that value by one.  As a result,
clients are always getting epoch/version-1 instead of epoch.

This manifested by a LibRadosTier.Promote test that failed becaues the
OSD had the OSDMap updates adding the tier and overlay but not the final
map change that set the cache-mode to writeback.  I suspect this is also
the cause of of spurious errors in the past where we've seen misdirected
request errors that made no sense.

Backport: emperor, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao@inktank.com>