Schedules the driver's next clean_up() query on 'ob' for 'time'
seconds from now, overriding the default idle-time rule (an object
is normally queried once it has gone unreferenced for the
'time to clean up' config setting). The deadline is one-shot: after
it fires, the object reverts to the idle-time rule.
If 'time' is omitted, any pending explicit deadline is cancelled and
the object reverts to the idle-time rule immediately.
In both forms the object is re-flagged for clean_up consideration
(like request_clean_up()), provided it defines a clean_up() apply.
Objects without a clean_up() apply are never queried.
Note that the sweep that performs clean_up queries runs every few
minutes, so 'time' is a lower bound, not an exact firing time. If
the 'time to clean up' config setting is 0, clean_up is disabled
globally and explicit deadlines are ignored as well.