BUG/MINOR: quic/server: free quic_retry_token on srv drop

BUG/MINOR: quic/server: free quic_retry_token on srv drop

A recent patch has implemented caching of QUIC token received from a
NEW_TOKEN frame into the server cache. This value is stored per thread
into a <quic_retry_token> field.

This field is an ist, first set to an empty string. Via
qc_try_store_new_token(), it is reallocated to fit the size of the newly
stored token. Prior to this patch, the field was never freed so this
causes a memory leak.

Fix this by using istfree() on <quic_retry_token> field during
srv_drop().

No need to backport.