JoinMarket: Add support for jmwalletd

[seberm]

This PR adds support for jmwalletd daemon which is needed for joinmarket-jam to run.

• The PR JoinMarket update and improvements #702 is prerequisite for this PR to review/merge.

Refs.:

• https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/JSON-RPC-API-using-jmwalletd.md
• https://joinmarket-org.github.io/joinmarket-clientserver/api/#operation/version

[seberm]

Hello @erikarvstedt @jonasnick ,
I am finallizing the patches for jmwalletd support, which is a prerequisite for the Jam (see PR #700).

This service should probably only be accessible locally. I don't see any reason to expose it via an onion address.

Additionally, please take a look at the address option, which I have marked as readOnly and internal. The reason for this is that I haven't found a way to set the listening address for jmwalletd. The service always listens on 0.0.0.0.

Also, do you think it makes sense to add some documentation about this service to the README?

Could you please review this and assist with the service hardening?

Thank you!

[naturallaw777]

Just wanted to checking in. What are the next steps or further organization to get this merged?

[naturallaw777]

Thanks @seberm for putting this together!