core: filetransaction: support unsafe visible file transaction

In some cases there are utilities that need to use the file
that is writing during transaction, this implies that we must
modify the original file.

In the unsafe visible mode we do exactly that, and revert by moving the backup
file to original file.

Change-Id: I66af1694922730a377257a907a9a80f4d071ec74
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>