Currently, accesses to the KEK store often assume that they are not acting concurrently. This can lead to TOCTTOU issues. Improve this by making the KEK store use atomic load/compare-and-swap semantics:

• The Keystore object itself loses its accessors and mutators of the keystore, and instead just hands out KeystoreSnapshots.
• These are versioned snapshots of the keystore.
• To modify the keystore, callers take a snapshot, modify it, then use the Keystore::commit function, which takes a modified KeystoreSnapshot and implements CAS semantics: The commit only goes through if the Snapshot's version matches the keystore's base version.