Configurable state cache size and enforce exact state cache size (#2314)

* Allow configuring state cache size via cli

* Track used cache size

* Expose memory counter to informant

* Enforce max cache size constraint exactly

* Default to 64 MiB

Co-Authored-By: cmichi <mich@elmueller.net>

* Remove self as parameter

substrate/core/service/test/src/lib.rs

@@ -112,6 +112,7 @@ fn node_config<F: ServiceFactory> (
 		keystore_path: root.join("key").to_str().unwrap().into(),
 		database_path: root.join("db").to_str().unwrap().into(),
 		database_cache_size: None,
+		state_cache_size: 16777216,
 		pruning: Default::default(),
 		keys: keys,
 		chain_spec: (*spec).clone(),