crimson/seastore: batch same-collection transactions

crimson/seastore: batch same-collection transactions

Previously, the per-collection ordering_lock provided both ordering and
single-holder exclusion, causing same-collection transactions to be serialized
one at a time.

Replace that with a per-collection queue and a single dispatch loop. When a
collection has a batch in flight, newly arriving transactions are queued.
The dispatch loop later drains the queue and merges transactions into one
SeaStore transaction (batch). Resulting in a one build, one ool_write and one journal record for the entire batch.

Only one batch is in flight per collection, so same-collection concurrency is
still prevented and ordering is preserved by the queue.

Callbacks are unchanged: FuturizedStore drains each caller's on_commit before
enqueueing, so only ops are merged. Each caller's promise is fulfilled when the
merged batch commits.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>