Refactor async callbacks and fix the double tlsdnsconnect callback

Refactor async callbacks and fix the double tlsdnsconnect callback

The isc_nm_tlsdnsconnect() call could end up with two connect callbacks
called when the timeout fired and the TCP connection was aborted,
but the TLS handshake was not complete yet.  isc__nm_connecttimeout_cb()
forgot to clean up sock->tls.pending_req when the connect callback was
called with ISC_R_TIMEDOUT, leading to a second callback running later.

A new argument has been added to the isc__nm_*_failed_connect_cb and
isc__nm_*_failed_read_cb functions, to indicate whether the callback
needs to run asynchronously or not.