remove unnecessary assertion in dns_dispatch_connect()
When a thread calls dns_dispatch_connect() on an unconnected TCP socket
it sets tcpstate
from DNS_DISPATCHSTATE_NONE
to _CONNECTING
.
Previously, it then INSISTed that there were no pending connections
before calling isc_nm_tcpdnsconnect().
If a second thread called dns_dispatch_connect() during that window of time, it could add a pending connection to the list, and trigger an assertion failure.
This commit removes the INSIST since the condition is actually harmless.
Closes #3456 (closed)