Reduce number of channel-log queries done by continuous-mode changes feed

Added a mode to MultiChangesFeed that lets it keep running indefinitely
instead of returning after it finds new entries. This lets it keep its
state around, so it will not have to check the channel logs more often
than it needs to.
Handle channel-log-cache requests for too-low sequence numbers

It's possible to tell that a starting sequence number is below any post
in the channel, and just send back the entire cache. This helps keep
the cache from getting messed up by the way the gateload test code
generates fake sequence IDs that often have sequence numbers lower than
any real entry in a channel.

Also, don't record a cache miss if there isn't really any channel log
document yet; this happens almost every time a new channel starts.