simplehttp async connection pool freeing

[ploxiln]

simplehttp async connection pool resources are never all freed, even if there is no connection activity for a long time. This can contribute to memory fragmentation and pin large heap regions, so even if the application using simplehttp async connections naturally frees all its other resources, it still has an RSS near its maximum.

Maybe async connection pool resources should be freed if all connections they are related to are done and gone for more than a few seconds.

[ploxiln]

There is indeed a function exposed to clean up all connection pool resources, typically before one's program exits, but it's not convenient to detect and handle quiet times and free and reinit the connection pool in one's program.