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/primitives/state-machine/src/lib.rs

@@ -40,6 +40,7 @@ mod overlayed_changes;
 mod proving_backend;
 mod trie_backend;
 mod trie_backend_essence;
+mod stats;
 
 pub use sp_trie::{trie_types::{Layout, TrieDBMut}, TrieMut, DBValue, MemoryDB};
 pub use testing::TestExternalities;
@@ -68,6 +69,7 @@ pub use trie_backend_essence::{TrieBackendStorage, Storage};
 pub use trie_backend::TrieBackend;
 pub use error::{Error, ExecutionError};
 pub use in_memory_backend::InMemory as InMemoryBackend;
+pub use stats::{UsageInfo, UsageUnit};
 
 type CallResult<R, E> = Result<NativeOrEncoded<R>, E>;