Merge bitcoin/bitcoin#21056: rpc: Add a -rpcwaittimeout parameter to limit time spent waiting

b9e76f1bf0 rpc: Add test for -rpcwaittimeout (Christian Decker)
f76cb10d7d rpc: Prefix rpcwaittimeout error with details on its nature (Christian Decker)
c490e17ef6 doc: Add release notes for the `-rpcwaittimeout` cli parameter (Christian Decker)
a7fcc8eb59 rpc: Add a `-rpcwaittimeout` parameter to limit time spent waiting (Christian Decker)

Pull request description:

  Adds a new numeric `-rpcwaittimeout` that can be used to limit the
  time we spend waiting on the RPC server to appear. This is used by
  downstream projects to provide a bit of slack when `bitcoind`s RPC
  interface is not available right away.

  This makes the `-rpcwait` argument more useful, since we can now limit
  how long we'll ultimately wait, before potentially giving up and reporting
  an error to the caller. It was discussed in the context of the BTCPayServer
  wanting to have c-lightning wait for the RPC interface to become available
  but still have the option of giving up eventually ([4355]).

  I checked with laanwj whether this is already possible ([comment]), and
  whether this would be a welcome change. Initially I intended to repurpose
  the (optional) argument to `-rpcwait`, however I decided against it since it
  would potentially break existing configurations, using things like `rpcwait=1`,
  or `rpcwait=true` (the former would have an unintended short timeout, when
  old behavior was to wait indefinitely).

  ~Due to its simplicity I didn't implement a test for it yet, but if that's desired I
  can provide one.~ Test was added during reviews.

  [4355]: https://github.com/ElementsProject/lightning/issues/4355
  [comment]: https://github.com/ElementsProject/lightning/issues/4355#issuecomment-768288261

ACKs for top commit:
  laanwj:
    Code review ACK b9e76f1bf0
  promag:
    ACK b9e76f1bf0.

Tree-SHA512: 3cd6728038ec7ca7c35c2e7ccb213bfbe963f99a49bb48bbc1e511c4dd23d9957c04f9af1f8ec57120e47b26eaf580b46817b099d5fc5083c98da7aa92db8638

doc/release-notes-21056.md

@@ -0,0 +1,6 @@
+New bitcoin-cli settings
+------------------------
+
+- A new `-rpcwaittimeout` argument to `bitcoin-cli` sets the timeout
+  in seconds to use with `-rpcwait`. If the timeout expires,
+  `bitcoin-cli` will report a failure. (#21056)