dnsdist: Prevent race while creating new TCP worker threads

dnsdist: Prevent race while creating new TCP worker threads

We try very hard to avoid using locks, but we need to prevent two
threads inserting into the TCP workers vector concurrently. While
this can't happen at runtime since the healthcheck thread is the
only one calling `g_tcpclientthreads->addTCPClientThread()`, this
might happen at startup time because we start the TCP acceptor
threads one after another and they all call it once.
This might result, for example, in one vector entry being overwritten
and another one remaining value-initialized to zero.