osd/SnapMapper: fix pacific legacy key conversion and introduce test

osd/SnapMapper: fix pacific legacy key conversion and introduce test

Octopus modified the SnapMapper key format from

  <LEGACY_MAPPING_PREFIX><snapid>_<shardid>_<hobject_t::to_str()>

to

  <MAPPING_PREFIX><pool>_<snapid>_<shardid>_<hobject_t::to_str()>

When this change was introduced, 94ebe0ea also introduced a conversion
with a crucial bug which essentially destroyed legacy keys by mapping them
to

  <MAPPING_PREFIX><poolid>_<snapid>_

without the object-unique suffix.  This commit fixes this conversion going
forward, but a fix for existing clusters still needs to be developed.

Fixes: https://tracker.ceph.com/issues/56147
Signed-off-by: Manuel Lausch <manuel.lausch@1und1.de>
Signed-off-by: Matan Breizman <mbreizma@redhat.com>