Use the delegation tree at the snapshot block

Currently, we use the most up-to-date delegation sets when creating the delegation graph.

Instead, we should use the delegations that were active at the snapshot block.

For this, we need to select a "main chain" that we use for the snapshot block. For other networks, we will need to use the timestamp of the snapshot block (from the "main chain").

After this, the delegation graph for a specific snapshot (block number) should only need to be computed once (and cached for later retrievals).

This will force us to store a lot more information (one delegation graph for every proposal). Related issue: https://github.com/gnosis/delegate-registry/issues/17.