#634: CouchDB writer

CouchDB will be far too slow for any large-scale setup. Inserts at
higher than 1k/sec will start to back up. Even MongoDB, which is
much, much faster than Couch, can only handle 2-3k/sec sustained. You
may want to include a warning in your documentation to only send
notice or a few other types of logs into CouchDB or risk loss.