ReplicatedPG: trim backfill intervals based on peer's last_backfill_started

ReplicatedPG: trim backfill intervals based on peer's last_backfill_started

Otherwise, we fail to trim the peer's last_backfill_started and get bug 11199.

1) osd 4 backfills up to 31bccdb2/mira01213209-286/head (henceforth: foo)

2) Interval change happens

3) osd 0 now finds itself backfilling to 4 (lb=foo) and osd.5
(lb=b6670ba2/mira01213209-160/snapdir//1, henceforth: bar)

4) recover_backfill causes both 4 and 5 to scan forward, so 4 has an interval
starting at foo, 5 has an interval starting at bar.

5) Once those have come back, recover_backfill attempts to trim off the
last_backfill_started, but 4's interval starts after that, so foo remains in
osd 4's interval (this is the bug)

7) We serve a copyfrom on foo (sent to 4 as well).

8) We eventually get to foo in the backfilling. Normally, they would have the
same version, but of course we don't update osd.4's interval from the log since
it should not have received writes in that interval. Thus, we end up trying to
recover foo on osd.4 anyway.

9) But, an interval change happens between removing foo from osd.4 and
completing the recovery, leaving osd.4 without foo, but with lb >= foo

Fixes: #11199
Backport: firefly
Signed-off-by: Samuel Just <sjust@redhat.com>