osd: capture hashing of objects to hash positions/pgs in pg_pool_t

The hashing is dependent on pool properties; capture (more of) it in a
method instead of having it in OSDMap.

Signed-off-by: Sage Weil <sage@inktank.com>
osdc/Objecter: expose methods for getting object hash position and pg

Signed-off-by: Sage Weil <sage@inktank.com>
librados: expose methods for calculating object hash position

Signed-off-by: Sage Weil <sage@inktank.com>
osd: add hit_set_* parameters to pg_pool_t

Add pool properties to control what type of HitSet we want to use, along with
some (mostly generic) parameters.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: add basic HitSet tracking

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

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