Get the current cache key URL, also referred to as the lookup URL. This must
be stored in a properly allocated URL object, typically created with a
This API can be called as early as
TS_HTTP_READ_REQUEST_HDR_HOOK but no later than
TS_HTTP_POST_REMAP_HOOK. This is the preferred and most efficient way to
modify the cache key, but an alternative is to use the old
TSCacheUrlSet(), which takes a simple string as argument.