hot market config changes

[chappjc]

To change market config, the dcrdex operator would issue a suspend, probably with a book purge, shutdown the process, edit the config, and restart dcrdex. The clients that were connected will reconnect and get the new config as per https://github.com/decred/dcrdex/pull/705.

To implement hot market config changes (i.e. dcrdex operator issues suspend, tweaks the config, triggers a dcrdex to reload config, and resumes market), the TradeResumption message should include the full msgjson.Market config for the resuming market. The client should also know to apply the new config of course. Ref: https://github.com/decred/dcrdex/pull/705#discussion_r496299161