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.

ethcore/light/src/on_demand/mod.rs

@@ -204,6 +204,8 @@ fn guess_capabilities(requests: &[CheckedRequest]) -> Capabilities {
 				caps.serve_headers = true,
 			CheckedRequest::HeaderByHash(_, _) =>
 				caps.serve_headers = true,
+			CheckedRequest::HeaderWithAncestors(_, _) =>
+				caps.serve_headers = true,
 			CheckedRequest::TransactionIndex(_, _) => {} // hashes yield no info.
 			CheckedRequest::Signal(_, _) =>
 				caps.serve_headers = true,