enhancement: refactor to maintain policy state in-memory

[cmaclaughlin]

Re: #25

The capacity scheduling plugin was used for frame of reference. A collection of policy information is kept in-memory on the PlacementPolicyManager. Instead of annotations, the policy info in the collection includes 2 sets:

• all pods that qualify for / fall under the policy
• all pods assigned to nodes in accordance with the policy ("managed by")

An event handler was attached to the pod informer so pod deletion can trigger updating the in-memory version of the policy accordingly. Additionally, the custom info object is included in the state object added to cycle state as a part of PreFilter and PreScore stages so that Filter and Score stages have access to it as well.