Fix max_keepalive_connections

[blazewicz]

Summary

This commit fixes a bug in both sync and async connection pools where idle connections were dropped from the pool even when max_keepalive_connections limit has not been reached. This happened because the check compared this setting's value to the total number of connections, not only of the idle ones.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.