osd/ReplicatedPG: archive hit_set if it is old and not full

This matches the condition under which we call _persist().

Signed-off-by: Sage Weil <sage@inktank.com>
osd/HitSet: fix up test instances for *Params

Signed-off-by: Sage Weil <sage@inktank.com>
osd/HitSet: track BloomHitSet::Params fpp in micros, not as a double

...and store it as a 32-bit value, so that it actually works!

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