types: add an Atomic64 type

Change-Id: I4677b4368b98e658b921697331d999b58de85d7f
fs: add .camli_fs_stats root directory; use it in tests.

Currently one a few stats are tracked, but more can be added easily now.

The only test using this is using it to verify that the read-only open optimization
is used.

Change-Id: I1e2fb8a0222ade82631670d7c51e684778d031ae