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.
The `IsOpen()` name is slightly confusing. In the above sequence we check if the file is "open" (by one definition) and then assert that it is not "open" (by another definition)