Refactor the customer to go through zkchannels-customer watch

[yaymukund]

This is a significant change, but ties into multiple issues we're facing:

• It's a prerequisite for Write spec for RPC Protocol #322
• It will allow us to resolve Update protocol to wait for merchant to announce when it has funded the channel #303
• It has a side effect of resolving Only allow new channels if the customer has a chain watcher up #241

The fix will happen in two parts:

1. zkchannel-customer watch should spin up a TCP server, and polling the chain should happen within that server.
2. All the commands should issue a command to the poll watcher.