MEDIUM: ring: always allocate properly aligned ring structures

MEDIUM: ring: always allocate properly aligned ring structures

The rings were manually padded to place the various areas that compose
them into different cache lines, provided that the allocator returned
a cache-aligned address, which until now was not granted. By now
switching to the aligned API we can finally have this guarantee and
hope for more consistent ring performance between tests. Like previously
the few carefully crafted THREAD_PAD() could simply be replaced by
generic THREAD_ALIGN() that dictate the type's alignment.

This was the last user of THREAD_PAD() by the way.