Pin the dns_dispatch to threads when reusing

Previously, dns_dispatch_gettcp() could pick a TCP connection created by different thread - this breaks our contractual promise to DNS dispatch by using the TCP connection on a different thread than it was created. Add .tid member to the dns_dispatch_t struct and skip the dispatches from other threads when looking up a TCP dispatch that we can reuse in dns_request.

NOTE: This is going to be properly refactored, but this change could be also backported to 9.18 for better stability and thread-affinity.