add metrics for rdb lru cache

[arnetheduck]

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%