crimson/os/seastore: simplify backref cache

crimson/os/seastore: simplify backref cache

Currently, the following transaction exec sequence would lead to
loss of backref:

1. Trans `A` merge a alloc backref for extent `X`
2. Trans `B` add a release backref for extent `X` to backref cache,
   during which it finds an in-cache alloc backref for extent `X` and
   decide not to add the release backref to cache
3. Trans `A` commit

In the above sequece, the release backref for extent `X` is lost.

This is a regression introduced when we try to optimize the backref cache.

This commit fix the issue by caching inflight backrefs in a multiset,
alloc/release ops that happen on the same paddr are queued in the order of
their happening. When doing gc, all those backrefs are merged.

Fixes: https://tracker.ceph.com/issues/56519
Signed-off-by: Xuehan Xu <xxhdx1985126@gmail.com>