wallet/walletrpc.c: txprepared transactions now use current tip blockheight by default.

[ZmnSCPxj]

Changelog-Changed: txprepare now prepares transactions whose nLockTime is set to the tip blockheight, instead of using 0. fundchannel will use nLockTime set to the tip blockheight as well.

[cdecker]

Do we really want yet another parameter, that confuses users?

Wouldn't it be more sensible adding the default to current blockheight in, and not exposing the option until people ask for it? Of course if you have a concrete use-case for this already I'll eat my words 😉

[niftynei]

If there's not a reason for wanting control over setting it explicitly, I'd suggest re-using the logic we use for withdraw, which mimic's the bitcoind logic also.

https://github.com/ElementsProject/lightning/blob/master/wallet/walletrpc.c#L331-L334

[ZmnSCPxj]

Well, we could use locktime and set it to 499999999 when doing the initial txprepare (the one that gets txdiscarded later) in fundchannel/multifundchannel, as a belt-and-suspenders protection to prevent accidentally txsending the initial txprepared transaction. Though input reservation would do just as well (if only there was a way to atomically get reserved UTXOs into a txprepare....)

@niftynei yes, that code just gets shifted around in this PR.

[ZmnSCPxj]

Removed locktime argument.

[cdecker]

Need to kick Travis a bit more, but LGTM

ACK 51c40f9