Add 30.x branch and/or v30.0 tag

[Sjors]

There's already some pull requests on Bitcoin Core that will change the interface:

• mining: getCoinbase() returns struct instead of raw tx bitcoin/bitcoin#33819
• interfaces: enable cancelling running waitNext calls bitcoin/bitcoin#33676

They are (as currently proposed, but debated...) not breaking changes, but they do change the .capnp files and none of these new methods will work with v30.

I'm also proposing some small breaking changes, timing TBD:

• ipc: breaking but non-urgent changes Sjors/bitcoin#106

Those change would work on master and v31+ and not with v30.

So it's probably a good idea to follow the Bitcoin Core branch and tag convention here:

• tag v30.0 since the release is out
• add a 30.x branch based on what's in master now
  • this only follows interface changes that Bitcoin Core backports (none so far)
• update master if and when any of the above pull requests are merged
• explain this structure in the README, maybe have the default instructions recommend the v30.0 tag and v30 Bitcoin Core release.

[plebhash]

cross referencing: bitcoin/bitcoin#33819 (comment)