osd/ReplicatedPG: put proxy read completion on finisher

We can't use the synchronous completion callbacks (in fast dispatch
context) do to the proxy read completion work.

Signed-off-by: Sage Weil <sage@redhat.com>
osd/ReplicatedPG: improve proxy read cancelation

Avoid taking the PG lock for a canceled read op (if we are lucky).  Recheck
after the lock is taken for good measure.

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