msggen: add batching method

[daywalker90]

Nothing special here, adds:

• batching

[cdecker]

This does not what it says on the tin:

The batching RPC command allows (but does not guarantee!) database
commitments to be deferred when multiple commands are issued on this RPC
connection. This is only useful if many commands are being given at once, in
which case it can offer a performance improvement (the cost being that if
there is a crash, it's unclear how many of the commands will have been
persisted).

Notice that this sets the current connection up to be batching, avoiding DB transactions being opened and closed for each following command. However, the cln-rpc crate uses a new JSON-RPC connection for each command, so this will set the connection to batching, but then immediately discard it again. There is no good concept of connection in grpc clients either, so this is a method that is not applicable outside of direct clients or plugins (for which this was actually created by the way).

We should document this (in a RPC comparison matrix? 🤔 ) and call it out as not applicable.

[cdecker]

This methods is not applicable except for local clients.

[daywalker90]

Interesting, i thought if you used e.g. .call() multiple times on the same ClnRpc variable it would be one and the same rpc connection. So are we not enabling this and instead documenting it somewhere?

[daywalker90]

Or just point out the difference between rpc and grpc?

[cdecker]

We should likely just not map this method through, and document somewhere the reason for it: it doesn't do what you'd expect.