This fixes a 'double_alloc' assertion in the following case:
- big_write object #1 v1 to block #100
- big_write object #1 v2 to block #101
- big_write object #2 v1 to block #100
Make syncs wait for all previous writes because it's the only way
to make sure that OSDs do not receive incomplete writes in LIST results
during peering when some writes are still in progress.
Also simplify blockstore submission queue logic.