If the btree updates pointing to a bucket were never flushed by the
journal before the bucket became empty again, we can reuse the bucket
without a journal flush.

This tweaks the tracking of journal sequence numbers in alloc keys to
implement this optimization: now, we only update the journal sequence
number in alloc keys on transitions to and from empty. When a bucket
becomes empty, we check if we can tell the journal not to flush entries
starting from when the bucket was used.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>