Better support for eth_getLogs in light mode (#9186)

* Light client on-demand request for headers range.

* Cache headers in HeaderWithAncestors response.

Also fulfills request locally if all headers are in cache.

* LightFetch::logs fetches missing headers on demand.

* LightFetch::logs limit the number of headers requested at a time.

* LightFetch::logs refactor header fetching logic.

* Enforce limit on header range length in light client logs request.

* Fix light request tests after struct change.

* Respond to review comments.

rpc/src/v1/helpers/errors.rs

@@ -101,6 +101,14 @@ pub fn request_rejected_limit() -> Error {
 	}
 }
 
+pub fn request_rejected_param_limit(limit: u64, items_desc: &str) -> Error {
+	Error {
+		code: ErrorCode::ServerError(codes::REQUEST_REJECTED_LIMIT),
+		message: format!("Requested data size exceeds limit of {} {}.", limit, items_desc),
+		data: None,
+	}
+}
+
 pub fn account<T: fmt::Debug>(error: &str, details: T) -> Error {
 	Error {
 		code: ErrorCode::ServerError(codes::ACCOUNT_ERROR),