dnsdist: Fix a crash in the Downstream TCP handler

dnsdist: Fix a crash in the Downstream TCP handler

when we are looking for an existing TCP connection to a backend to
reuse, we routinely (every 60s by default) clean up existing
connections from the cache.
7b5f590ee72fecf54c0c40b24e98ba03a406af53 removes a connection
from the cache more aggressively when it has failed, but I did not
notice that the same function might be called from the cache cleaning
algorithm. It caused the cache cleanup function to call this function
which in turns tried to remove the connection from the same cache,
invalidating the iterator of the cache algorithm, and causing a crash
when the function returned.