Parallelize funding RPC if channel open > 1

[DanGould]

we don't need repeat rpc calls, which I think we're doing now.

lnd: https://api.lightning.community/#batchopenchannel

cln: https://lightning.readthedocs.io/lightning-multifundchannel.7.html

[Kixunil]

batchopenchannel doesn't allow PSBT funding flow so it's incompatible with PayJoin.

[DanGould]

In that case we could still make The LndClient Sync and Send so the "Verify funding" logic can interact with remote lightning nodes in parallel lnd::LndClient::verify_funding as demonstrated in #10

[Kixunil]

Oh, yes, definitely! I was thinking about it before. It was POC code after all. :)