Convert ASYNC code to use new Thread API

The async code uses thread local variables. We should convert to using
the new Thread API for doing this.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/async/arch/async_null.h

@@ -65,13 +65,9 @@ typedef struct async_fibre_st {
 } async_fibre;
 
 
-# define async_set_ctx(nctx)                    0
-# define async_arch_get_ctx()                   ((async_ctx *)NULL)
 # define async_fibre_swapcontext(o,n,r)         0
 # define async_fibre_makecontext(c)             0
 # define async_fibre_free(f)
 # define async_fibre_init_dispatcher(f)
-# define async_get_pool()                       NULL
-# define async_set_pool(p)                      0
 
 #endif