Ensure alignment of 64 bit integers on 32 bit systems

Technically this is only required for 64 bit integers being accessed
with the atomic package but it seemed like a good practice to put all
64 bit integers at the beginning of the structure to ensure they are
aligned.

https://code.google.com/p/go/issues/detail?id=6404
Merge pull request #729 from mozilla-services/alignment_64bit

Ensure alignment of 64 bit integers on 32 bit systems