Capacity gossip is now gossiped per store, not by group.

This should be sufficient for the beta stages. When more stores
are added, then switch to gossiping capacity through groups,
so that only the top N will be gossiped. This will lower the gossip
and storage load on all machines.
Simple, linear implementation of a store finder for allocation decisions

Store now tracks gossip keys of store capacity, and scans the capacity gossip
for attribute matches when they need to make allocation decisions. A simple,
    linear implementation that has a straighforward way to implement garbage
    collection of capacity keys, but faster methods can be employed later. It
    is likely sufficent as long as it is suffient for all stores to gossip
    capacity, but will need to be reevaluated when capacity is gossiped through
    groups.