dnsdist: Cleanup closed TCP downstream connections

dnsdist: Cleanup closed TCP downstream connections

Regularly walk the TCP downstream connections to properly close
whose that have been shutdown by the other end. It occurs only
after a TCP client connection has been closed and at most every
`setTCPDownstreamCleanupInterval()` seconds, defaulting to 60s.
Until now we only detected that the other end closed the connection
when we tried to reuse it. While this is not an issue with a small
number of backends because the connection are reused pretty quickly,
with a large number of backends dnsdist might end up with thousands
of idle TCP connections to downstream servers in `CLOSE_WAIT` state,
wasting open file descriptors.