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/client/src/light/backend.rs

@@ -183,6 +183,10 @@ impl<S, F, Block, H> ClientBackend<Block, H> for Backend<S, F, H> where
 		&self.blockchain
 	}
 
+	fn used_state_cache_size(&self) -> Option<usize> {
+		None
+	}
+
 	fn changes_trie_storage(&self) -> Option<&Self::ChangesTrieStorage> {
 		None
 	}