Fix situation on a heterogeneous memory cluster where the order of

constraints mattered in a job.

    Details include:
    A job doesn't request memory but the system is running
    with CR_*MEMORY with no default memory limit and the job requests nodes
    with features of different sizes.  Previously the order of constraints
    mattered where the smaller memory node would need to be requested first
    or the job would fail.

    Bug 2608