osd: track the number of hit_set archive objects in a pg

Also, use this value in agent_choose_mode instead of the max
number.

Related: #8124
Signed-off-by: Samuel Just <sam.just@inktank.com>
ECMsgTypes::ECSubWrite: fix at_version indentation

Signed-off-by: Samuel Just <sam.just@inktank.com>
encoding: use unqualified name for encode/decode in boost::optional encoding

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: propogate hit_set history with repop

We don't actually send the whole info on each repop, just the log
entries, updated stats, and a few other bits.  For hit_set ops, we need
to also communicate the new hit_set history status atomically with the
log entries and the transaction.  Thus, we add a channel for an optional
pg_hit_set_history_t field in PGBackend::submit_transaction interface
and associated messages and implementations to update the hit_set info
field along with the log entries.

This also means that hit_set_(persist|trim) update an
updated_hit_set_history field on the OpContext instead of directly
modifying the info field.

Fixes: #8124
Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,PGLog: update hit_set during peering

Fixes: #8124
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::agent_load_hit_sets: take ondisk_read_lock

Otherwise, the hit_set might be not yet written due to a recently
completed recovery.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: do not use shard for hit_set object names

Signed-off-by: Samuel Just <sam.just@inktank.com>