Tracking ticket for follow-on modifications related to the generic TCP connection pool, per discussion in #3747:

• refactor HTTP1 connection pool to extend it
• modify tcp_proxy to use connection pool vs. tcpConnForCluster
• modify redis_proxy to use connection pool

Other possible lines of inquiry:

• moving tcp_proxy drain manager into tcp conn pool (or perhaps cluster, per tcp_proxy: convert TCP proxy to use TCP connection pool #3938 (comment))