implement agent communication manager

The role of this manager is synchronizing communication to agents. Currently it is allowed to do 2 requests in parallel to an agent but it will not work as agent processes requests one by one. So communication with agents should be organized on server side. More in https://gitlab.isc.org/isc-projects/stork/-/wikis/Designs/Agent-Server-Communication#agent-tasks-manager. Current implementation is pretty simple: all requests are passed via channel to the manager which sends them one by one to agents.