Explore using the upstream implementation of ChannelPool instead of AHC's one

[TomGranot]

ChannelPool, as it is implemented in AHC, predates the Netty implementation introduced in 2015 (see this AHC user guide entry from 2012 in which ConnectionPool is referenced as proof).

As the Netty release mentions, connection pooling in the world of Netty-based clients is a valuable feature to have, one that jfarcand implemented himself at the time.

Need to explore the tradeoffs of stripping away the current implementation and swapping it for the upstream one for simplicity (? - TBD).