Added/fixed a few XML comments
Disallow moving a directory to a subdirectory of itself
Added an override on FdbOperationContext that takes a non-async Action<...>
Breaking Change 3 of 3: CancellationTokens considered harmful.

- Completely rewritten the cancellation story by removing most optional CancellationToken from methods on FdbTransaction
- Added the IFdbTransactional interface that expose the ReadAsync, WriteAsync and ReadWriteAsync retry methods.
- FdbTransaction instances managed a single CancellationToken that is used internall by all async methods (GetXXX, Commit, OnError, ...)
- App code can get this the token when calling async code by doing "await SomeFileOrSocketIOMethod(..., tr.Token)"
- The Token can be set either by calling BeginTransaction(...) (by default uses the db's token) or by passing it to the transactional methods (ReadAsync, WriteAsync, ReadWriteAsync)
- FdbOperationContext is now embedded inside the transaction instance and can be obtained via "tr.Context"
- Directory Layer: Fixed bug in MoveAsync()