Keep-alive strategies for connections in connection pool

[weissi]

Much like grpc-swift already does, in large deployments it's often important to make sure that the HTTP client has for example 1,000 connections ready to go straight away.

Currently, we can't force 1,000 connections to be pre-opened (one problem) but even if we allow AHC to open 1k connections and make it open 1k connections by just creating enough traffic, AHC will close them again. After the idleTimer expires. And we can't just raise the idleTimer to say 8 hours because that usually triggers TCP inactivity timeouts so the connections will be cut from elsewhere.

AHC should support different keep-alive strategies such as:

• TCP keepalives
• sending a HEAD / request once every 5 mins or so