Add `--bitcoin-rpcclienttimeout` CLI parameter

[cdecker]

As discussed in https://github.com/ElementsProject/lightning/pull/2780 we should likely expose this option through the command line so that users can adapt it to their needs.

Open questions:

• @ZmnSCPxj proposed subtracting the --bitcoin-retry-timeout (better yet, just take the maximum of the two), to make sure we actually trigger an error at the expected time, rather than --bitcoin-retry-timeout + --bitcoin-rpcclienttimeout

[BitcoinJiuJitsu]

@s373nz is keen to have a run at this bounty after discovering on Sphinx, can we assign @cdecker?

[s373nZ]

@cdecker @BitcoinJiuJitsu I believe I've got some local code to define and passthrough the argument to bcli.

In terms of the open question, are you saying that --bitcoin-rpcclienttimeout should be modified to max(bitcoin_retry_timeout, bitcoin_rpcclienttimeout)? Based on my reading of #2778, that might still cause a long wait time as per the default values and your comment here suggests a lower value:

https://github.com/ElementsProject/lightning/issues/2778#issuecomment-506960780

From reviewing the conversations in #2780 and #2778, my reading is that we want the min(). Am I misunderstanding?

[s373nZ]

@cdecker @BitcoinJiuJitsu I took a first shot at this in #7095. The earlier question was because I initially set the parameter up with a default value of 900 to match bitcoind. I removed that and set the default to 60, where the suggestion to take the max() makes much more sense.

[cdecker]

Very nice, thanks @s373nZ, I'll review the PR as soon as possible :+1: