* ProcessTree: add core process tree logic
* make Step implicitly called by Handle* methods
* lint
* naming convention
* widen pidversion to be generic
* move os specific backfill to os specific impl
* simplify ts checking
* retain/release a whole vec of pids
* document processtoken
* lint
* namespace
* add process tree to project-wide unit test target
* case change annotations
* case change annotations
* remove stray comment
* default initialize seen_timestamps
* fix missing initialization of refcnt and tombstoned
* reshuffle pb namespace
* pr review
* move annotation registration to tree construction
* use factory function for tree construction
