osd/ReplicatedPG: load older HitSets into memory

If our evict_mode is non-idle, load older HitSets into memory in the agent
work thread.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: EOPNOTSUPP on hit_set_get on non-replicated pools

We need to use the async read stuff for this to work.

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