[9.20] fix: usr: Fix TLS contexts cache object usage bug in the resolver

:iscman:`named` could terminate unexpectedly when reconfiguring or
reloading, and if client-side TLS transport was in use (for example,
when forwarding queries to a DoT server). This has been fixed.

Closes #5653

Backport of MR !11295

Merge branch 'backport-5653-tlsctx_cache-reference-bug-fix-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11299