Potential Block Fetch logic optimisations

[mrBliss]

In #1054 we already made some optimisations to the Block Fetch logic so that it no longer stands out in the profiling report.

Duncan has the following optimisations in mind whenever it shows up again in the profiling report:

• Check that the Block Fetch decision code is not being woken up too frequently.
• Put a minimum time between decisions, so even if we do wake up again promptly, we wait a few milliseconds, i.e. rate limiting.