Add comment about request canceling to tests
Update test deployment target to 5.1
Started request objects for canceling operations, connection timeouts, failureBlock refactoring

- all API methods return a request, with a cancel method
- DLSFTPRequest retains a cancel handler block
- When cancelled, the block is executed.  The block may close the socket (for connection canceling)
- Added failWithErrorCode to remove repeated queuedFailureBlock dispatching
- notes about dispatch_data_apply