Make `graphman prune` prune on an ongoing basis

[lutter]

Running graphman prune <subgraph> <offset> will change a subgraph so that it will retain about offset many blocks going back from the subgraph head. Pruning will therefore be an ongoing operation with the guarantee that the subgraph can always be queried at least offset many blocks from the subgraph head, though at times it might retain more history than that.

Requires #3665