Summary:
The C++ standard doesn't guarantee alignment of things bigger than std::max_align_t which is typically 16 bytes. This means that to ensure something is the only thing on a cache-line we need padding on each side of it to exclude anything else.

CachelinePadded<T> will now be larger than it was before, and reinterpret_cast to/from T is no longer valid.

Reviewed By: yfeldblum

Differential Revision: D5380849

fbshipit-source-id: 20f275c875eb4bede4aef19ac7224913ce9b6d51