Add public API with a request object for Select/Update/Upsert

This patch provides request types for part of space operations: Select,
Update and Upstream. It allows to create requests step by step. The
main idea here is too provide more extensible approach to create
requests.

Part of #126

Author: oleg-jukovec

CHANGELOG.md

@@ -35,6 +35,7 @@ CI and documentation.
 - queue-utube handling (#85)
 - Master discovery (#113)
 - SQL support (#62)
+- Add public API with a request object for Select/Update/Upstream (#126)
 
 ### Changed