dnsdist: Limit # of proxy protocol-enabled outgoing TCP connections

dnsdist: Limit # of proxy protocol-enabled outgoing TCP connections

TCP worker threads keep a cache of outgoing TCP connections to a
backend to be able to reuse them for subsequent queries. Proxy
protocol-enabled outgoing TCP connections are trickier because the
proxy protocol payload is sent only once at the beginning of the
TCP connection, contains the source and destination addresses and
ports, and thus the connections can only be reused with the exact
same incoming TCP connection. For this reason these connections are
stored in a specific structure of the incoming connection, instead
of the TCP worker connection cache. However, we can only reuse a
given proxy protocol-enabled outgoing TCP connection for a subsequent
query if the TLV values contained in the proxy-protocol payload
associated to the new query are exactly the same than the ones
associated to the existing query. Up until now, we would keep an
unbounded amount of proxy protocol-enabled connections around if
the TLV values were, for example, randomly assigned per query.
This commit sets a limit on the number of such connections we will
keep around: we will keep at most N connections, where N is the
ratio between the number of concurrent queries on a single TCP
connection supported by the backend and the number of concurrent
queries on a single TCP connection supported by the frontend, with
a hard cap to 5.