'flogtool filter' as a network pipe

[Brian Warner]

Good idea from zooko: make the 'flogtool filter' command (which currently takes a file and produces a new smaller file) accept a logport.furl and produce a logport.furl . Then, once the filters described in #53 are in place (and the gatherer tap from #56 too), we could efficiently follow a subset of the log events by doing:

• configure the gatherer with a #56 tap
• run 'flogtool filter' on the same host as the gatherer, with a #53 filter expression
• run 'flogtool tail' on your remote host, pointing at the filter's logport

This would be an easier-to-implement point along the way to the remote-filtering-expressions that I want to build in #57.