osd/OSD: optimize send_incremental_map

osd/OSD: optimize send_incremental_map

Previosuly, if `since` was earlier than the oldest_map,
only the lastest full osdmap would be sent.
However, we could actually send inc/full maps from oldest_map
up to the current epoch.

Moreover, build_incremental_map_msg is suited to handle this case as well.
if `since` is earlier than `cluster_osdmap_trim_lower_bound` it
will be adjusted accordingly.
At any point, the cluster_osdmap_trim_lower_bound is later (or equal) to
the superblock's oldest_map. See: past_intervals.rst (OSDSuperblock::maps)
Hence, if since < oldest_map, and oldest_map <= clutser_lower_bound
then since < cluster_lower_bound.

Note: oldest_map stands as a mark to indicate up until which epoch
      the current OSD trimmed its osdmaps. See OSD::trim_maps().
      Meaning, the OSDMaps in the range of [oldest_map, trim_lower_bound]
      may be at hand to be shared. Nevertheless, we should actually use
      the cluster_trim_lower_bound to set the correct range.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>