0.1.1: Keep-Alive sockets housekeeping

[slvrtrn]

Summary

This is a potential fix for #150.

• Introduce a mechanism to detect a potentially expired socket on the client side when using Keep-Alive. If such socket is detected, and retry is enabled in the configuration, both socket and request will be immediately destroyed, and the client will try again.
• Allow disabling Keep-Alive on the client side entirely.
• TRACE log level added

It'd be nice to have a freeSocketTimeout setting to remove expired idling sockets in the background before we even try to use them (such as in agentkeepalive). Still, even there, it was not working as I expected.

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

[mshustov]

LGTM