Multiselect integration test

Testing of #111 uncovers that protocol negotiation happens more often than in the master branch. Moreover, that negotiation often fails due to unexpected messages which may indicate the error-prone messages flow and order.

The issue is to create some integration test for the multiselect protocol to reveal such errors and verify messages order correctness without any dependency on underlying implementation (either callbacks or tasks scheduling - #111 addresses it).

Log example of multiselect issues when #111 gets employed:

[2021-03-15 15:29:45.873680000][th:584649][info] multiselect got a unexpected request-response combination - sending 'ls'
[2021-03-15 15:29:45.873701000][th:584649][warning] listener_mgr can not negotiate protocols, other side has violated a protocol and sent an unexpected message
[2021-03-15 15:29:45.984183000][th:584649][info] multiselect got a unexpected request-response combination - sending 'ls'
[2021-03-15 15:29:45.984203000][th:584649][warning] listener_mgr can not negotiate protocols, other side has violated a protocol and sent an unexpected message

Review issue #111 alongside the multiselect protocol flow and any existing integration-test entry points. Reproduce the logged unexpected message and negotiation-order failures, then define a test that verifies message ordering without depending on callbacks or task scheduling.