fixup, restore freeing jsonrpc separately before ld, lesson learned

Before this, below error happens:
https://github.com/ElementsProject/lightning/runs/4097724359?check_suite_focus=true

Valgrind error file: valgrind-errors.77564
==77564== Invalid write of size 8
==77564==    at 0x1843C9: list_del_ (list.h:327)
==77564==    by 0x1858E0: destroy_invoice_waiter (invoices.c:570)
==77564==    by 0x220AC7: notify (tal.c:240)
==77564==    by 0x220FDE: del_tree (tal.c:402)
==77564==    by 0x221030: del_tree (tal.c:412)
==77564==    by 0x221030: del_tree (tal.c:412)
==77564==    by 0x221030: del_tree (tal.c:412)
==77564==    by 0x22137A: tal_free (tal.c:486)
==77564==    by 0x148212: main (lightningd.c:1214)
==77564==  Address 0x63391b0 is 64 bytes inside a block of size 88 free'd
==77564==    at 0x483CA3F: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==77564==    by 0x2210AC: del_tree (tal.c:421)
==77564==    by 0x221030: del_tree (tal.c:412)
==77564==    by 0x221030: del_tree (tal.c:412)
==77564==    by 0x22137A: tal_free (tal.c:486)
==77564==    by 0x148212: main (lightningd.c:1214)
==77564==  Block was alloc'd at
==77564==    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==77564==    by 0x220B35: allocate (tal.c:250)
==77564==    by 0x2210F6: tal_alloc_ (tal.c:428)
==77564==    by 0x184A7C: invoices_new (invoices.c:142)
==77564==    by 0x187166: wallet_new (wallet.c:86)
==77564==    by 0x147C77: main (lightningd.c:994)

Invoice waiter is allocated off cmd, i.e. off ld->jsonrpc->commands
but the list head wallet->invoices->waiters is allocated off wallet.
So if jsonrpc happened to be freed after wallet, then the
list head ptr is already gone and list_del doesn't know.

There are many instances where list head is allocated off ld
but element's (content) is allocated off the command.

lightningd/jsonrpc.h

@@ -22,7 +22,7 @@ enum command_mode {
 /* Context for a command (from JSON, but might outlive the connection!). */
 /* FIXME: move definition into jsonrpc.c */
 struct command {
-	/* Off json_cmd->commands */
+	/* Off ld->jsonrpc->commands */
 	struct list_node list;
 	/* The global state */
 	struct lightningd *ld;

lightningd/lightningd.c

@@ -1194,6 +1194,9 @@ int main(int argc, char *argv[])
 	/* Tell plugins we're shutting down, closes the db for write access. */
 	shutdown_plugins(ld);
 
+	/* Cleanup JSON RPC separately: destructors assume list pointers off ld */
+	tal_free(ld->jsonrpc);
+
 	/* Clean our our HTLC maps, since they use malloc. */
 	htlc_in_map_clear(&ld->htlcs_in);
 	htlc_out_map_clear(&ld->htlcs_out);