lib-sql: Add API support for asynchronously iterating over rows. sql_query() can already do an async lookup, but the full result needs to be available immediately. This can be inefficient for large results. Add a new SQL_RESULT_NEXT_MORE return value and sql_result_more() for asynchronously requesting more results. This changes the API a bit, but isn't done by default by any drivers yet. Also callers that can't handle this are hopefully checking for "ret < 0", which allows them to handle such an async-more request as an error instead. sql_result_next_row() will be changed to return enum in a separate commit to keep backwards compatibility in v2.2.x.

--- a/src/lib-sql/sql-api-private.h	Thu Jul 06 12:40:33 2017 +0300
+++ b/src/lib-sql/sql-api-private.h	Mon Jul 17 14:22:35 2017 +0300
@@ -126,6 +126,8 @@
 	const char *const *(*get_values)(struct sql_result *result);
 
 	const char *(*get_error)(struct sql_result *result);
+	void (*more)(struct sql_result **result, bool async,
+		     sql_query_callback_t *callback, void *context);
 };
 
 struct sql_field_map {