Use hiredis for the dashboard's Redis connection.

hiredis is recommended for use when the Redis connection may deal with
large datasets, e.g. results from ZRANGE* queries.  That describes the
dashboard pretty well (i.e. Job#read_new_entries) so we might as well
give it a shot.

The rationale is "performance", but I wonder if that also includes
"memory usage".
Replace LogAnalyzer with a few smaller scripts.

The cogs program is suffering from inexplicable memory leaks and/or
process bloat.  (I can't tell which one is actually happening.  Hell,
both might be occurring.)

This commit, and indeed this branch, are an attempt to rewrite cogs in a
memory-conscious way.  The programs here are meant to be glued together
with e.g. socat, ucspi-{tcp,unix,...} and operate on streams without
uncontrolled object instantiation.  (Possible exception: the
zrangebyscore call in analyze-logs.)

I've run these programs on the production ArchiveBot Redis updates
channel, and have noticed much more acceptable memory usage over time.
Next step: disable the LogAnalyzer actor, replace its duties, and see
just how we do.