Improve computational efficiency of the upgrade signalling mechanism

[cmwaters]

Summary

The new upgrading mechanism is relatively naive in it's implementation, tallying signalling votes every block. There are two ways we can improve this:

• Delete the votes after an upgrade so we don't have to iterate over the entire set (this is a relatively minor improvement if we don't expect to tally too often)
• Tally periodically. Either:
  • every n blocks, say 10,000
  • using a "crank" message, where anyone can pay to count the votes and trigger the version change once a quorum has been reached.

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