Fix some bugs that only occur rarely

Testing running the bot for long periods of time (an hour or more),
issues appeared with the stream implementation and with the URL matching
regex.
Use a REPLACE call to insert comments

The REPLACE call won't get upset if we try to record a comment twice.
It'll just update it for us. This is good because we might encounter
comments multiple times as we crawl reddit. The performance of this call
is roughly the same as a simple INSERT.