Fix out of sync checkpoint files in simulations sharing state

Fix out of sync checkpoint files in simulations sharing state

When multidir simulations share the state, the checkpoint files
of the different simulations should all be from the same step.
To ensure this, MPI barriers have been added before renaming
the checkpoint files from their temporary to their final names.
So now the contents can never be out of sync. In the worst, and
rather unlikely, case that something going wrong during renaming,
some checkpoint files could have temporary and some final names.

Refs #2440.

Change-Id: I88088abb726a36dbf9a9db2fa2eb4a46c3bf2cd7