Usability improvements

[jakinyele]

• [ ] Clarify how much time for establish and close in README for testnet (re: 10 mins on average and 20 mins worst case)
• [ ] Add helper script for starting/stopping the customer and merchant daemons (e.g., zkchannels-local.sh start|stop)
• [ ] Add logging levels so that the block info is not necessarily outputted by default - a user may just be interested in the zkchannel operation progress. Also it’d be useful to have notification of which part of the process is undergoing, the customer or the merchant verification. Related issue: https://github.com/boltlabs-inc/zeekoe/issues/91