Add an UPDATE flag that instructs the server to return immediately from UPDATE

Add an UPDATE flag that instructs the server to return immediately from UPDATE, without waiting for wal_writer to finish.

Implementation details
-----------------------------

Flag name: BOX_NOWAIT
We need to spin off a separate fiber that will handle the client connection going forward, since the original fiber still needs to wait for the wal_writer, and rollback the transaction in memory if wal_writer fails.

We need to learn how to live with multiple tuple versions in memory for this to work well.

See also box-wait-replica flags.