joinmarket: Add option `settings`

[erikarvstedt]

Joinmarket settings can now be freely specified:

services.joinmarket = {
  enable = true;
  settings = {
    POLICY = {
      merge_algorithm = "gradual";
      tx_fees = 5;
    };
    LOGGING = {
      console_log_level = "DEBUG";
    };
  };
};

Closes #707

The updated joinmarket-ob-watcher is deployed at https://nixbitcoin.org and is working fine.

@seberm, can you help reviewing and testing this?

[erikarvstedt]

I've removed option txfee_contribution. It's deprecated and should no longer be used: https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/v0.9.3/docs/release-notes/release-notes-0.9.3.md

[erikarvstedt]

I think we can merge this now.