dnsdist: Fix TCP clients threads vector and counters initialization

dnsdist: Fix TCP clients threads vector and counters initialization

By tracking the FD leak reported in #3300, I observed that:
* we could create up to g_maxTCPClientThreads TCP threads,
but the corresponding vector size was hardcoded at 1024
(which the default for g_maxTCPClientThreads)
* the counters were not explicitely initialized

This commit fixes that and adds some additional checks to make
sure we don't add more TCP client threads, as that could lead to
a race if the vector is resized.