mon/OSDMonitor: set POOL_CREATING flag until initial pool pgs are created

mon/OSDMonitor: set POOL_CREATING flag until initial pool pgs are created

Set the flag when the pool is created, and clear it when the initial set
of PGs have been created by the mon.  Move the update_creating_pgs()
block so that we can process the pgid removal from the creating list and
the pool flag removal in the same epoch; otherwise we might remove the
pgid but have no cluster activity to roll over another osdmap epoch to
allow the pool flag to be removed.

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