osd/ReplicatedPG: respect RWORDERED rados flag

If this flag is set, we need to order reads as writes.  In particular, this
means that reads will wait for degraded object recovery even if there is a
local copy.  And subsequently will be ordered after a preceding write that
is waiting for the same thing.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 9322305c80e995e1c4a964edff0fc094329d951b)