Get information about how many client connections is usually in progress

As was understood in INDY-1686, ZMQ may consume a lot of memory if there are a lot of simultaneous client requests, since it uses round-robin to read client queues.

Plenum does (be default) 100 reads in one looper run.

We need to test if situation with memory consumption improves when we restrict the number of client connections.

Acceptance criteria

• Get information about how many client connections is usually in progress:
  • Include metrics to track current number of open client connections (a PR)
  • Run two tests with metrics enabled: 10 NYMs per sec and 100 NYMs per sec. Compare results and how many connections are used.