Remember writes as "unsynced" only after completing them

Previously BS_OP_SYNC could take unfinished writes and add them into the journal before
they were actually completed. This was leading to crashes with the message
"BUG: Unexpected dirty_entry 2000000000001:9f2a0000 v3 unstable state during flush: 338"

src/blockstore_sync.cpp

@@ -24,6 +24,7 @@ int blockstore_impl_t::continue_sync(blockstore_op_t *op, bool queue_has_in_prog
     if (PRIV(op)->op_state == 0)
     {
         stop_sync_submitted = false;
+        unsynced_big_write_count -= unsynced_big_writes.size();
         PRIV(op)->sync_big_writes.swap(unsynced_big_writes);
         PRIV(op)->sync_small_writes.swap(unsynced_small_writes);
         PRIV(op)->sync_small_checked = 0;