- [x] add `in_buildcache` field to DB records to indicate what parts of an index, 
       which includes roots and dependencies, are in the buildcache.
- [x] add `mark()` method to DB for setting values on single nodes of the DAG.
		
	
- [x] add `in_buildcache` field to DB records to indicate what parts of an index, 
       which includes roots and dependencies, are in the buildcache.
- [x] add `mark()` method to DB for setting values on single nodes of the DAG.