plugins/sql: add payment_hash index to channelmoves table.

This significantly speeds up the query which bookkeeper often does:

		      "SELECT created_index"
		      " FROM channelmoves"
		      " WHERE payment_hash = X'%s'"
		      "   AND credit_msat = %"PRIu64
		      "   AND created_index <= %"PRIu64,

On large databases this scan is expensive, and a payment_hash index
cuts it down a great deal.  It does take longer to load the channelmoves
in the first place though (about 3x).

Before:
	$ while sleep 10; do wc -l /tmp/bkpr-progress; done
	169505 /tmp/bkpr-progress
	196010 /tmp/bkpr-progress
	219370 /tmp/bkpr-progress
	235671 /tmp/bkpr-progress
	244242 /tmp/bkpr-progress
	255362 /tmp/bkpr-progress
	265636 /tmp/bkpr-progress
	276966 /tmp/bkpr-progress
	284451 /tmp/bkpr-progress
	288836 /tmp/bkpr-progress
	296578 /tmp/bkpr-progress
	304571 /tmp/bkpr-progress

After:
	$ while sleep 10; do wc -l /tmp/bkpr-progress; done
	161421 /tmp/bkpr-progress
	238273 /tmp/bkpr-progress
	281185 /tmp/bkpr-progress
	305787 /tmp/bkpr-progress

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

Author: rustyrussell

plugins/sql.c

@@ -197,6 +197,10 @@ static const struct index indices[] = {
 		"channelmoves",
 		{ "account_id", NULL },
 	},
+	{
+		"channelmoves",
+		{ "payment_hash", NULL },
+	},
 };
 
 static enum fieldtype find_fieldtype(const jsmntok_t *name)
@@ -1757,20 +1761,22 @@ static const char *fmt_indexes(const tal_t *ctx, const char *table)
 	for (size_t i = 0; i < ARRAY_SIZE(indices); i++) {
 		if (!streq(indices[i].tablename, table))
 			continue;
-		/* FIXME: Handle multiple indices! */
-		assert(!ret);
+		if (!ret)
+			ret = tal_fmt(ctx, " indexed by ");
+		else
+			tal_append_fmt(&ret, ", also indexed by ");
 		BUILD_ASSERT(ARRAY_SIZE(indices[i].fields) == 2);
 		if (indices[i].fields[1])
-			ret = tal_fmt(tmpctx, "`%s` and `%s`",
-				      indices[i].fields[0],
-				      indices[i].fields[1]);
+			tal_append_fmt(&ret, "`%s` and `%s`",
+				       indices[i].fields[0],
+				       indices[i].fields[1]);
 		else
-			ret = tal_fmt(tmpctx, "`%s`",
-				      indices[i].fields[0]);
+			tal_append_fmt(&ret, "`%s`",
+				       indices[i].fields[0]);
 	}
 	if (!ret)
 		return "";
-	return tal_fmt(ctx, " indexed by %s", ret);
+	return ret;
 }
 
 static const char *json_prefix(const tal_t *ctx,