Adjust subgraph deployment assignment cost dynamically

[Jannis]

Right now we're costing every subgraph identically, however, some (e.g. ENS) are much more resource intensive than others (e.g. Decentraland). As more subgraphs get added to a node, this might lead to scaling problems.

This metric could be based on factors like:

• number of entities created / modified
• number of events processed
• amount of time required for past handlers

[lutter]

See #940 for one way we could use to measure subgraph cost.

[lutter]

We've completely changed our strategy for assigning subgraphs, and this ticket doesn't make much sense any more.