graph gc built on replicated dict
Merge branch 'master' of https://github.com/neilconway/garbage-collection
Merge branch 'master' of https://github.com/neilconway/garbage-collection
Merge branch 'master' of https://github.com/neilconway/garbage-collection
Merge branch 'master' of https://github.com/neilconway/garbage-collection
simple graph gc with epochs
epoch stuff