dnsdist: Add an option to limit the number of queued TCP connections

dnsdist: Add an option to limit the number of queued TCP connections

By default, if all TCP threads are busy we will queue incoming TCP
connections until the communication pipes are full.
In some setup, we might run out of file descriptors quickly,
causing new TCP connections but also health checks, web requests,
console connections to fail.
This new `setMaxTCPQueuedConnections()` option limits the number of
queued connections, dropping new ones when the threshold is
reached.