add state_getKeysPaged (#4718)

* add storage_getNextKey

* RPC storage_getKeysPages

* respect count

* use iterator

* improve

* add tests

* improve

* add doc comments

* Make prefix optional

* update error

* improve

substrate/client/rpc-api/src/state/error.rs

@@ -41,6 +41,14 @@ pub enum Error {
 		/// Details of the error message.
 		details: String,
 	},
+	/// Provided count exceeds maximum value.
+	#[display(fmt = "count exceeds maximum value. value: {}, max: {}", value, max)]
+	InvalidCount {
+		/// Provided value
+		value: u32,
+		/// Maximum allowed value
+		max: u32,
+	},
 }
 
 impl std::error::Error for Error {
@@ -63,6 +71,11 @@ impl From<Error> for rpc::Error {
 				message: format!("{}", e),
 				data: None,
 			},
+			Error::InvalidCount { .. } => rpc::Error {
+				code: rpc::ErrorCode::ServerError(BASE_ERROR + 2),
+				message: format!("{}", e),
+				data: None,
+			},
 			e => errors::internal(e),
 		}
 	}