= GC properties

1. the execution time of the GC should be linked to
active dataset (ie. where the number of references have changed)
but not to global dataset (for scalability purpose)

2. GC should run on active dataset

 2.1. GC should not delete data being referenced by a pending process or
still referenced

 2.2. GC should be idempotent: 2 concurrent or sequential runs should
not have a different outcome than a single one

3. GC should remove data from the underlying store

 3.1. an unreferenced piece of data should be removed after 1 day

 3.2. less than 10% of unreferenced data of a significant dataset
should persist

4. GC should report what it does

