Initial checkin.
Break up scanning, fetching & loading.
Separate NNTP client worker.
NNTP connection pooling.

Separate per-newsgroup indexers from the connection. Instead, there is a
pool of NNTP client workers that can grab article headers for any
indexer.
Log errors in mongo, pending header counter.
Article header parsing.

Beginnings of article clustering into collections.
Add test for article parsing.
Fix NNTP client workers' RPC-style channel usage.