Add config "collector.combiner.min-days-back" to filter events that are too new

Goal: Combiner instance to handle old events coming in late
Add handling of "collector.combiner.min-days-back" setting

Default is 0 / off
Code changes per feedback:

1) Deprecate "collector.combiner.max-days-back" and replace with "collector.combiner.days-ago-to-start"
2) Rename new arg to match (1), collector.combiner.days-ago-to-end
3) Validate start > stop days.  Wonky due to lack of builtin cross field validation
4) Force secondary validation on StoredObjectCombiner per request
Add explicit test against combiner end date
Add collector.combiner.days-ago-disabled setting

Optional setting to disable the combiner start/end date filtering altogether
(collector.combiner.days-ago-to-start / collector.combiner.days-ago-to-end).

This doesn't expose the setting to StoredObjectCombiner constructor
except when passing a ServerConfig object
Fix typo in bounds used for test
Add the 'disabled' property to the deprecated property test
Rename variables to reflect 'newer' date status
Merge pull request #20 from gwittel/combiner-min-days-back

Add combiner configuration for min days back filtering