So far we use a single muto to guard three different datastructures
inside class authz_ctx: the authz HANDLE, the user context HANDLE
and the context cache list. Split the single muto into three
independent SRWLOCKs and guard all datastrcutures as necessary to
avoid thread contention.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>