database: pull out database code into a new module

We're going to reuse the database controllers for the accounting plugin

wallet/db_queries_postgres.c

@@ -0,0 +1,13 @@
+#include "config.h"
+#include "db_postgres_sqlgen.c"
+
+#if HAVE_POSTGRES
+
+struct db_query_set postgres_query_set = {
+	.name = "postgres",
+	.query_table = db_postgres_queries,
+	.query_table_size = ARRAY_SIZE(db_postgres_queries),
+};
+
+AUTODATA(db_queries, &postgres_query_set);
+#endif /* HAVE_POSTGRES */