Make block polling dynamic to the current network

Different EVM networks have different block-times. Currently we use the same fixed polling interval for all custom networks.

Instead, it would be better if we even did a very brief rolling average of recent block times to derive a reasonable polling interval.

The issue names no files, tests, or entry points. Start by locating the fixed polling interval used for custom EVM networks and inspect how recent block times can be sampled; done means polling adapts to a brief rolling average for each network.

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