DiskFile already exposes a reader method that creates the DiskFileReader
instance. Add a writer method for parity.
DiskFile currently only provides a context manager create - that will
open and close the DiskFileWriter. Add explicit open and close methods
to support more flexibility in how callers manage life-cycle on their
DiskFileWriter instances.
Diskfile confusingly manages some state for DiskFileWriter (e.g. fd,
tmppath, use_linkat). Encapsulate the DiskFileWriter state to improve
readability and reduce coupling (e.g. put_succeeced).
Change-Id: If18e0041680470a9c57a08e9ea9327acba8593df
33c7650753