HTTP/3: handled insertion reference to a going to be evicted entry.

HTTP/3: handled insertion reference to a going to be evicted entry.

As per RFC 9204, section 3.2.2, a new entry can reference an entry in the
dynamic table that will be evicted when adding this new entry into the dynamic
table.

Previously, such inserts resulted in use-after-free since the old entry was
evicted before the insertion (ticket #2431).  Now it's evicted after the
insertion.

This change fixes Insert with Name Reference and Duplicate encoder instructions.