lightningd: wrap htlc replay in a database transaction.

AFAICT this only "worked" previously because replay htlc simply failed
them all (no peers are currently connected).  With upcoming changes
(foreshadowed by the comment) this is no longer true:

Attempting to prepare a db_stmt outside of a transaction: wallet/invoices.c:373
lightningd: FATAL SIGNAL 6 (version v0.7.3-188-g45b0af4-modded)
0x55b475590a73 send_backtrace
	common/daemon.c:41
0x55b475590b1d crashdump
	common/daemon.c:54
0x7f16c557b46f ???
	???:0
0x7f16c557b3eb ???
	???:0
0x7f16c555a898 ???
	???:0
0x55b475564c8f fatal
	lightningd/log.c:814
0x55b4755c3ed5 db_prepare_v2_
	wallet/db.c:605
0x55b4755c76b5 invoices_find_unpaid
	wallet/invoices.c:373
0x55b4755ce91c wallet_invoice_find_unpaid
	wallet/wallet.c:1990
0x55b47555861f invoice_check_payment
	lightningd/invoice.c:257
0x55b475557a7c htlc_add_set
	lightningd/htlc_set.c:112
0x55b47557b294 handle_localpay
	lightningd/peer_htlcs.c:332
0x55b47557c63c htlc_accepted_hook_callback
	lightningd/peer_htlcs.c:857
0x55b475585573 plugin_hook_call_
	lightningd/plugin_hook.c:118
0x55b47557c747 plugin_hook_call_htlc_accepted
	lightningd/peer_htlcs.c:882
0x55b47557ca3e peer_accepted_htlc
	lightningd/peer_htlcs.c:991
0x55b47557ffb9 htlcs_resubmit
	lightningd/peer_htlcs.c:2131
0x55b4755620f7 main
	lightningd/lightningd.c:801

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

lightningd/lightningd.c

@@ -797,8 +797,11 @@ int main(int argc, char *argv[])
 
 	/*~ Process any HTLCs we were in the middle of when we exited, now
 	 * that plugins (who might want to know via htlc_accepted hook) are
-	 * active. */
+	 * active.  These will immediately fail, since no peers are connected,
+	 * however partial payments may still be absorbed into htlc_set. */
+	db_begin_transaction(ld->wallet->db);
 	htlcs_resubmit(ld, unconnected_htlcs_in);
+	db_commit_transaction(ld->wallet->db);
 
 	/*~ Activate connect daemon.  Needs to be after the initialization of
 	 * chaintopology, otherwise peers may connect and ask for