The local store could end up in a state where the writer is
closed but the reference is locked after a commit on an
existing object.
Cleans up Commit logic to always close the writer even after
an error occurs, guaranteeing the reference is unlocked after commit.
Adds a test to the content test suite to verify this behavior.
Updates the content store interface definitions to clarify the behavior.
Signed-off-by: Derek McGowan <derek@mcgstyle.net>