MINOR: session: release a few other pools when stopping

MINOR: session: release a few other pools when stopping

We currently release all pools when a proxy is stopped, except the
connection, pendconn, and pipe pools. Doing so can improve further
reduce memory usage of old processes, eventhough the connection struct
is quite small, but there are a lot and they can participate to memory
fragmentation. The pipe pool is very small and limited, and not exported
so it's not done here.