Hide the file descriptor and disk write methodology for PUTs

Towards moving the DiskFile class into place as the API definition for
pluggable DiskFile backends, we hide the file descriptor and the
method of writing data to disks. The mkstemp() method has been renamed
to writer(), and no longer returns an fd, but a new object that
encapsulates the state tracked for writes. This new object is then
used directly to perform the reminder of the write operations and
application of required semantics.

Change-Id: Ib37ed37b34a2ce6b442d69f83ca011c918114434
Signed-off-by: Peter Portante <peter.portante@redhat.com>