utils: add a global tmpctx.

I did a brief audit of tmpctx uses, and we do leak them in various
corner cases.  Fortunely, all our daemons are based on some kind of
I/O loop, so it's fairly easy to clean a global tmpctx at that point.

This makes things a bit neater, and slightly more efficient, but also
clearer: I avoided creating a tmpctx in a few places because I didn't
want to add another allocation.  With that penalty removed, I can use
it more freely and hopefully write clearer code.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/subdaemon.c

@@ -73,6 +73,8 @@ void subdaemon_setup(int argc, char *argv[])
 	secp256k1_ctx = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY
 						 | SECP256K1_CONTEXT_SIGN);
 
+	setup_tmpctx();
+
 	for (int i = 1; i < argc; i++) {
 		if (streq(argv[i], "--log-io"))
 			logging_io = true;