WIP: Standalone mode

[hashbrowncipher]

@jolynch: this branch implements something we had discussed a little while ago, namely a standalone or "oneshot" execution mode for Synapse. This is for situations where you would like to know what information is in Synapse's backing datastore, but you don't want to read it out of the datastore directly in order to preserve separation of concerns. By adding this standalone operation, we can use the same code paths that Synapse would use to read the data.

My initial use-case is monitoring scripts. I would like to deploy a container with the Synapse gem, and run it as a script that produces a list of backends as output. My monitoring script will process this output, check it for sanity, and report issues to my monitoring system if the results are insane.

This change makes use of a new method on watchers, that I've called read. As far as I can tell, right now watchers don't implement any method which provides the list of backends as output without mutating other state. read is designed for that purpose.

So far I've just touched the Zookeeper watcher. Please review for initial feasibility.

Example invocation:

synapse_standalone.rb '{"method":"zookeeper","path":"/nerve/services/postgres_main_read","hosts":["10.74.4.104:2181","10.74.4.70:2181","10.74.1.43:2181"]}'