Normalize sums prior to exporting them

[dashpole]

This is based on the normalizesumsprocessor, but has a few differences:

• Timeseries are garbage-collected if they are not used after 20 minutes to prevent it from leaking memory
• It converts non-monotonic sums to gauges instead of keeping track of the latest and inserting resets when the value decreases.
• It handles explicit reset points (where start time is after end time).

To implement garbage collection, i've extracted the cache out into its own package.