messages/MOSDMap: Rename oldest_map to cluster_osdmap_trim_lower_bound

messages/MOSDMap: Rename oldest_map to cluster_osdmap_trim_lower_bound

Previously, MOSDMap messages sent to other OSDs were populated with the
superblocks's oldest_map. We should, instead, use the superblock's
cluster_osdmap_trim_lower_bound because oldest map is merely a marker
for each osd's trimming progress.

As specified in the docs:
***
We use cluster_osdmap_trim_lower_bound rather than a specific osd's oldest_map
because we don't necessarily trim all MOSDMap::oldest_map. In order to avoid
doing too much work at once we limit the amount of osdmaps trimmed using
``osd_target_transaction_size`` in OSD::trim_maps().
For this reason, a specific OSD's oldest_map can lag behind
OSDSuperblock::cluster_osdmap_trim_lower_bound
for a while.
***

Signed-off-by: Matan Breizman <mbreizma@redhat.com>
(cherry picked from commit 721c9aea1869bf95a167b8879756b4f71fbf41bf)