osd: make OSD a config observer

Signed-off-by: Sage Weil <sage@inktank.com>
osd: make osd_max_backfills dynamically adjustable

Signed-off-by: Sage Weil <sage@inktank.com>
filestore: disable extra committing queue allowance

The motivation here is if there is a problem draining the op queue
during a sync.  For XFS and ext4, this isn't generally a problem: you
can continue to make writes while a syncfs(2) is in progress.  There
are currently some possible implementation issues with btrfs, but we
have not demonstrated them recently.

Meanwhile, this can cause queue length spikes that screw up latency.
During a commit, we allow too much into the queue (say, recovery
operations).  After the sync finishes, we have to drain it out before
we can queue new work (say, a higher priority client request).  Having
a deep queue below the point where priorities order work limits the
value of the priority queue.

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