crimson/osd: rework handling of MOSDAlive.

crimson/osd: rework handling of MOSDAlive.

1. Migrate `MOSDAlive` crafting from `OSD` to `ShardServices`.
2. `send_alive()` takes the `want` epoch from callers
   instead of assuming it matches current OSDMap.
   Callers pass `pg_history_t::same_interval_since`.
   The passed epoch is handled similarly to `queue_want_up_thru()`
   in the classical.
   The message is sent once and when the desired epoch is greater
   than the one returned by `OSDMap::get_up_thru()`.
3. Send the message also when completing the `PeeringEvent`.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>