transaction: make releasefn a private attribute
This is a handle on a callable that is called when the journal
is closed. The value is specified at __init__ time. It doesn't
need to be exposed on the public interface. So mark it as private.
Differential Revision: https://phab.mercurial-scm.org/D4626
transaction: make file a private attribute
This holds a file handle for the journal file. This file handle
should not be touched outside the journal class and doesn't
belong on the public interface.
Differential Revision: https://phab.mercurial-scm.org/D4625