Created by: Louis-Ye
This commit is a supplement for PR #3050.
We need to clean postcall_cache on drwrap_exit as well, otherwise
post-callback (one of the arguments when calling drwrap_wrap_ex) will not be invoked at re-attach. This is because the registration of post-callback relies on pre-callback, and pre-callback checks postcall-cache before registering the post-callback. Stale data in postcall_cache prevents post-callback being registered to the hash table.
Fixes #3049 (closed)