Add connection pooling to http-netty4

[alzimmermsft]

Unlike Reactor Netty, Netty itself doesn't have a built-in connection pooling mechanism for HTTP clients that talk to many different endpoints. The built-in ChannelPool implementations assume the Bootstrap is tied to a single remote address.

For optimal performance, we'll want to introduce our own handling the supports Channel reuse. But it will require a few changes and new APIs.

1. Channel reuse MUST support many remote addresses. And for each remote address the pool must support zero or more pooled Channels.
2. Redesign of our custom ChannelHandler implementations may need rework to support having multiple request-response flows.
3. Optional: Support configurability of the Channel pool, such as the number of pooled Channels allowed, how long the Channel can remain unused, etc.