Major Description
Add ability to carry an opaque context in docTimer, similar to cronTimer so users can supply additional information when registering docTimers that is passed to the callback when the timer fires.
Old signature:
docTimer(func_callback, timestamp, doc_id);
New signature:
docTimer(func_callback, timestamp, doc_id, [payload_context]);
Payload can be limited to some reasonable value, say 500kB. If it exceeds this, the timer creation should fail.
