Use epoch subgraph to get network specific epoch start blocks

[fordN]

Now that the protocol has migrated to Arbitrum One we need to update the algorithm for getting epoch start blocks.

The arbitrum network subgraph tracks epoch start blocks using L1 block numbers, so we can no longer rely on it for this information. Instead, the epoch subgraph will need to be used to identify the start block for an epoch, i.e the block that POIs should be generated when closing an allocation in that epoch.