PGLog::IndexedLog::trim(): rollback_info_trimmed_to_riter may be log.rend()

Fixes: #9731
Backport: giant, firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit d458b4f0f31161f561ff98e58ed979cf20c6f588)
PG::choose_acting: in mixed cluster case, acting may include backfill

Fixes: 9696
Backport: firefly, giant
Introduced: 92cfd370395385ca5537b5bc72220934c9f09026
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 9b18d99817c8b54e30dff45047dfe1b29871d659)
Merge remote-tracking branches 'upstream/wip-9696-firefly' and 'upstream/wip-9731-firefly' into wip-sam-firefly-testing