Urgent bugfix upgrade for makers, and JSON RPC API

IF YOU RUN A MAKER BE SURE TO READ THE FIRST SECTION OF THE RELEASE NOTES.

Please read the release notes at https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/release-notes/release-notes-0.9.3.md for details on what this release includes.

This release fixes a bug for makers using the relfee option where they could potentially lose up to the txfee (or txfee_contribution) sats in a coinjoin. See above for details. There is also a number of more minor improvements, and notably a new script jmwalletd.py that allows you to run a JSON-RPC server to allow a client to control your Joinmarket wallet (this is for UI development, mostly).

You're strongly recommended to backup, and then recreate your joinmarket.cfg (in ~/.joinmarket by default), to make sure you have the latest version of the config settings. Then read the above release notes which will guide you on what changes may or may not be needed for your situation (in this case, note you're advised to change the txfee value in the YIELDGENERATOR config).

Please verify the PGP signatures of the releases (zip files and exe file, see below) before using. They are signed with AdamISZ's pubkey (fingerprint: 2B6F C204 D9BF 332D 062B 461A 1410 01A1 AF77 F20B.

All releases before this (so 0.9.2 and earlier) should be considered deprecated for Makers (or yieldgenerator runners) because of the above-described bug.

A reminder that since 0.7.0, (gpg signed) Windows executable builds of JoinmarketQt (for Takers) are included with the release.

(Similar binaries for Linux and Mac are not provided but may be if people show interest in testing and working on it.)