libmn: optimize WorkerContext * TLS

`libmn::platform::TLS` was too high-level for a platform
specific interface. It was forcing us to store a pointer
to a pointer for the `WorkerContext *` TLS. Expose the raw
pthread_{get,set}specific interface and move the original
TLS template class to a platform independent file. Now
the `WorkerContext *` TLS stores the actual pointer so we
drop one level of pointer indirection. This is a lot faster
since this is a common lookup.
sigaltstack_fiber: use lower-level TLS interface

As in, 00a1f3d6fe48b8d5095fba55eb3bf8b2526136f6, since the FiberStorage
TLS is just a pointer, drop down to a more low-level interface to
avoid an extra pointer indirection.