HitSet: add an is_full() method

Use it in ReplicatedPG instead of looking at the pg_hitset_info_t::target_size.

Signed-off-by: Greg Farnum <greg@inktank.com>
pg_hit_set_info_t: remove unused size, target_size members

Signed-off-by: Greg Farnum <greg@inktank.com>
ReplicatedPG: do not seal() HitSets until we're done with them

We don't want to seal HitSets just because we're writing a
snapshot to disk; it potentially shrinks the in-memory one
we want to keep adding stuff to!

Signed-off-by: Greg Farnum <greg@inktank.com>
HitSet: do not permit users to seal() more than once

As the name implies, you should only be able to seal() once. Add
a sealed member (which we encode) and assert if seal() gets called
more than once.

Signed-off-by: Greg Farnum <greg@inktank.com>