[9.20] fix: dev: Disassociate the SSL object from the cached SSL_SESSION

When the SSL object was destroyed, it would invalidate all SSL_SESSION
objects including the cached, but not yet used, TLS session objects.

Properly disassociate the SSL object from the SSL_SESSION before we
store it in the TLS session cache, so we can later destroy it without
invalidating the cached TLS sessions.

Closes #4834

Backport of MR !9271

Merge branch 'backport-4834-detach-SSL-from-cached-SSL_SESSION-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9274