osd/ReplicatedPG: add basic HitSet tracking

Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Farnum <greg@inktank.com>
osd/ReplicatedPG: use vectorized osd_op outdata for pg ops

This lets us put PGLS in a compound operation.  Nothing does that yet, but
this would allow it.
Despite appearances, this is not a protocol change and does not require
a feature bit for clients: using the osd_ops vector mechanisms store all
the data in the same places as before, it just fills in some of the
already-decoded-but-empty data structures in the MOSDOpReply header.
<Greg note:> We may need a feature bit to let clients know they can send
compound PG ops to OSDs, though? Or maybe we can let it be covered
by supporting hitset ops.

Signed-off-by: Sage Weil <sage@inktank.com>
librados, osd: list and get HitSets via librados

Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Farnum <greg@inktank.com>
librados: create new ceph_test_rados_api_tier target

Move the dirty/undirty test to it, and add one for HitSets.

Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Farnum <greg@inktank.com>
osd/HitSet: only compress if density is < 50%

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