[9.20] fix: dev: Attach socket before async streamdns_resume_processing

[9.20] fix: dev: Attach socket before async streamdns_resume_processing

Call to `streamdns_resume_processing` is asynchronous but the socket
passed as argument is not attached when scheduling the call.

While there is no reproducible way (so far) to make the socket reference
number down to 0 before `streamdns_resume_processing` is called, attach
the socket before scheduling the call. This guard against an hypothetic
case where, for some reasons, the socket refcount would reach 0, and be
freed from memory when `streamdns_resume_processing` is called.

Closes #5620

Backport of MR !11247

Merge branch 'backport-5620-attach-socket-streamdns_resume_processing-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11260