and that should fix it. The oldestMulti value in pg_control would get updated by itself some time later. If you experience stalls before oldestMulti fixes itself, you could stop the server (cleanly!) and then pg_resetxlog -m x,y where x is the correct nextMulti value from pg_controldata and y is 20783.
Are you referreing to NextMultiXactId of current pg_controldata ( from postgres 9.3.5) for x?
There is also NextMultiOffset, not sure If you meant to reset this. Please confirm.
Thanks,
Dinesh