osd/: add sync/async read methods to PGBackend

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: use pgbackend->objects_read_sync() as appropriate

Signed-off-by: Samuel Just <sam.just@inktank.com>
src/: add CEPH_OSD_OP_READ_SYNC

EC pools won't support SYNC reads.  This means that TMAP and friends
along with reads from an object class will not be supported.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: support async reads on ec pools

Signed-off-by: Samuel Just <sam.just@inktank.com>