osd: Allow multiple objects with same version in missing list.

osd: Allow multiple objects with same version in missing list.

Most of the time, a single version in a PG can only correspond to a single object.

However, following a PG merge it is possible, even likely, that two objects will
have the same version.   The PG Log works around this by discarding the log.

However, during backfill, it is possible for the missing list to be build with
these duplicate versions.

A recently added assert detected that this scenario was corrupting the reverse
missing list (rmissing). This behaviour has always existed, but was previously
unnoticed.  It could cause some bugs and potentially loop-asserts on OSDs,
although mostly would not be noticed.

Here we fix this properly, by converting rmissing to a multimap. This is wrapped
in some insert functions, which assert that the rmissing list does not end up
with duplicate entries.  The code is optimised for the case where there are no
duplicate versions.

Additionally, some of the old asserts have been rolled into the insert functions.

Fixes: https://tracker.ceph.com/issues/75778
Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>
(cherry picked from commit f3940400952b444a31f59b633fa3fa35437c87a9)