File class members are encapsulated, and internal lock was added.

A lock was added to the file class to properly handle multiple threads accessing the self encryptor. Actual chunk storage and incrementing is still a work in progress.

This technically completes JIRA ticket MAID-153 too.
What if `IsOpen()` was renamed `HasBuffer()` (and `VerifyHasBuffer()`)? I think that might be a better description.