feat(bitcoind): pass the current known block height

When core lightning is asking the information about
the blockchain with `getchaininfo` command lightningd
know already the information about the min and max block height.

the problem is when we have a smarter Bitcoin backend that is able
to switch between different clients in some cases is helpful
give the information about current known height by lightningd and
pass it down to the plugin.

In this way, the plugin knows what is the correct known height from lightnind, and can
try to fix some problems if any exit.

This is particularly useful when you are syncing a new backend from scratch
like https://github.com/cloudhead/nakamoto and we avoid returning the
lower height from the known, and avoid the crash of core lightning.

With this information, the plugin can start to sync the chain and return
the answer back only when the chain is in sync with the current status of
lightningd.

Another reason to add this field and not wait the correct block in core
lightning itself is because Bitcoin Core is extremely slow to sync up,
so the question here is, how long should we wait? The time depends
on various factors.

With this approach of informing the plugin about the height, in some cases,
you can start the syncing but move the execution to another backend until
the previous one is ready.

The problem I want to solve is that I don't want to be left in the dark when
we run `getchaininfo`, and I want to have the opportunity to wait for
the blockchain sync or decide to dispatch the request elsewhere.

Changelog-Added: Pass the current known block height down to the getchaininfo call.
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

plugins/bcli.c

@@ -656,7 +656,15 @@ static struct command_result *getchaininfo(struct command *cmd,
                                            const char *buf UNUSED,
                                            const jsmntok_t *toks UNUSED)
 {
-	if (!param(cmd, buf, toks, NULL))
+	/* FIXME(vincenzopalazzo): Inside the JSON request,
+         * we have the current height known from Core Lightning. Therefore,
+         * we can attempt to prevent a crash if the 'getchaininfo' function returns
+         * a lower height than the one we already know, by waiting for a short period.
+         * However, I currently don't have a better idea on how to handle this situation. */
+	u32 *height UNUSED;
+	if (!param(cmd, buf, toks,
+		   p_req("last_height", param_number, &height),
+		   NULL))
 		return command_param_failed();
 
 	start_bitcoin_cli(NULL, cmd, process_getblockchaininfo, false,