FIXME Sync only completed writes

Currently it's impossible because it leads to errors similar to

terminate called after throwing an instance of 'std::runtime_error'
  what():  BUG: Unexpected dirty_entry 1000000000001:29480000 v65540 unstable state during flush: 0x151

Probably because in that case flushers should wait for previous writes too

src/blockstore_impl.cpp

@@ -163,20 +163,10 @@ void blockstore_impl_t::loop()
             }
             else if (op->opcode == BS_OP_SYNC)
             {
-                // wait for all small writes to be submitted
-                // wait for all big writes to complete, submit data device fsync
+                // sync only completed writes?
                 // wait for the data device fsync to complete, then submit journal writes for big writes
                 // then submit an fsync operation
-                if (has_writes)
-                {
-                    // Can't submit SYNC before previous writes
-                    continue;
-                }
                 wr_st = continue_sync(op);
-                if (wr_st != 2)
-                {
-                    has_writes = wr_st > 0 ? 1 : 2;
-                }
             }
             else if (op->opcode == BS_OP_STABLE)
             {