Design: Add support for storing historical facts

One of the short-to-medium term PE capabilities that wagner is interested in is adding a filter dialog to the Event Inspector UI that allows users to filter events based on the facts that exist on the node where the event occurred.

One complicating factor is that we currently only store the most recent set of facts for a node, but Event Inspector is capable of showing events that occurred on previous agent runs. Thus, correlating the events against the latest set of facts could result in incorrect / misleading data in the case where the value of a fact has changed since the time the event occurred. We have a few different options:

1. Ignore the possible inconsistencies;
2. Restrict the filtering to a well-known set of facts that is extremely unlikely to change
3. Store historical facts

There are other upcoming UI features (mostly related to inventory and showing how facts have changed over time) that will more definitively rely on historical facts, so I believe this work will need to be done at some point regardless; it's just a matter of prioritization.

kenn aaron wagner : this ticket should probably be linked as a blocker to any specific UI feature tickets that depend on it, whenever those land.

This ticket is intended to cover only the storage of historical facts; I'll link in some related tickets about querying them.