librados: add wait_for_latest_map()

There are times when we need to make sure the client has the latest osdmap,
for example after sending a mon command modifying pool properties.

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>