db: Enable foreign keys and cascade on shachain delete

Should make maintenance a bit easier.

wallet/db.c

@@ -31,7 +31,7 @@ char *dbmigrations[] = {
        num_valid INTEGER,			 \
        PRIMARY KEY (id));",
     "CREATE TABLE shachain_known (                                      \
-       shachain_id INTEGER,			 \
+       shachain_id INTEGER REFERENCES shachains(id) ON DELETE CASCADE,	\
        pos INTEGER,							\
        idx INTEGER,							\
        hash BLOB,							\
@@ -155,6 +155,10 @@ static struct db *db_open(const tal_t *ctx, char *filename)
 	tal_add_destructor(db, close_db);
 	db->in_transaction = false;
 	db->err = NULL;
+	if (!db_exec(__func__, db, "PRAGMA foreign_keys = ON;")) {
+		fatal("Could not enable foreignkeys on database: %s", db->err);
+	}
+
 	return db;
 }