crimson/os/seastore/segment_cleaner: add dedicated backref trimming process

crimson/os/seastore/segment_cleaner: add dedicated backref trimming process

Space reclamation needs to merge backrefs up to the point where the latest
release of extents within the scope of the reclamation process happened.
When the journal size is large, that merge may generate a transaction
record with size exceeds the max record size threshold. So we need have a
backref trimming process that merge most of the backrefs before the space
reclamation happens.

This commit also fixes issue: https://tracker.ceph.com/issues/55692, by
repeating the inflight backrefs trimming transaction when it's
invalidated by other trans on the ROOT block

Fixes: https://tracker.ceph.com/issues/55692
Signed-off-by: Xuehan Xu <xxhdx1985126@gmail.com>