State-db I/o metrics (#4562)

* add usage mod to sp-state-machine

* State usage tracking finalized.

* fix spaces

* add license preamble

* imporove output

* review suggestions

* update naming

* merge fixes

* Update client/db/src/light.rs

Co-Authored-By: Gavin Wood <gavin@parity.io>

Co-authored-by: Gavin Wood <github@gavwood.com>

substrate/client/db/src/light.rs

@@ -580,6 +580,9 @@ impl<Block> LightBlockchainStorage<Block> for LightStorage<Block>
 				writes: io_stats.writes,
 				reads: io_stats.reads,
 				average_transaction_size: io_stats.avg_transaction_size() as u64,
+				// Light client does not track those
+				state_reads: 0,
+				state_reads_cache: 0,
 			}
 		})
 	}