Add "sched_weight" logic

Add 64-bit sched_weight (scheduling weight) to node_set struct
Populate it with the node's weight (possibly reboot weight)
plus high-order bits for FLEX-reservation and rebooting.
No longer are node weights of INFINITE or (INFINITE-1) used
to flag FLEX or reboot requirements so we don't need to
worry about overlapping node weight values. It also will
cleanly allow the cons_tres plugin to be passed ALL usable
nodes at one time to accumulate resources from the lowest
weight first and only use individual higher weight nodes
as needed (rather than possibly using mostly higher
weight nodes).
bug 4821