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>

lightningd/test/run-key_derive.c

@@ -1,5 +1,4 @@
 #define SUPERVERBOSE
-static void *tmpctx;
 
 #include <assert.h>
 #include <ccan/str/hex/hex.h>
@@ -23,7 +22,7 @@ int main(void)
 	struct secret base_secret, per_commitment_secret;
 	struct pubkey base_point, per_commitment_point, pubkey, pubkey2;
 
-	tmpctx = tal_tmpctx(NULL);
+	setup_tmpctx();
 	secp256k1_ctx = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY
 						 | SECP256K1_CONTEXT_SIGN);