This is a first step towards measuring the efficiency of the LRU caches
over time - metrics can be collected during import or when running
regulary.
Since `nim-metrics` carries some overhead for its default way of
reporting metrics, this PR implements a custom collector over atomic
counters, given that this is one of the hottest spots in the block
processing pipeline.
Using a compile-time flag, the same metrics can be printed on exit which
is useful when comparing different strategies for caching - here's a
recent run over blocks 16000001-1616384 - this is a good candidate to
expose in a better way in the future, maybe:
```
state vtype miss hit total hitrate
Account Leaf 4909417 4466215 9375632 47.64%
Account Branch 20742574 72015123 92757697 77.64%
World Leaf 940483 1140946 2081429 54.82%
World Branch 8224151 131496580 139720731 94.11%
all all 34816625 209118864 243935489 85.73%
```
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