Feature/topic polling

Directly solves CH-328 (initial message slow)
Partly solves CH-21 (whitelisting)

Whitelisting involves solving two problems:
1. sending to targets (topics) that don't exist.
2. sending a payload type to target that it doesn't understand.

This checkin resolves a big chunk of that first one.  The second doesn't seem to be a severe and may not actually need to be implemented...

The implementation solution involves polling nsqlookupd for an active list of topics and keeping that list up to date.