MEDIUM: connection: use pool-conn-name instead of sni on reuse

MEDIUM: connection: use pool-conn-name instead of sni on reuse

Implement pool-conn-name support for idle connection reuse. It replaces
SNI as arbitrary identifier for connections in the idle pool. Thus,
every SNI reference in this context have been replaced.

Main change occurs in connect_server() where pool-conn-name sample fetch
is now prehash to generate idle connection identifier. SNI is now solely
used in the context of SSL for ssl_sock_set_servername().