Fix KvListOperation with {start,after}Key offset

`KvListOperation` are currently broken with `{start,after}Key` offsets due to the way query parts are concatenated in `HttpClient.java`, https://github.com/orchestrate-io/orchestrate-java-client/blob/master/src/main/java/io/orchestrate/client/HttpClient.java#L352-L359

Example query with `limit` set to `3` and `startKey` set to `helloworld`: `limit=3startKeyhelloworld`.

This PR adds proper separation of the key/value pairs.

Bonus: I've added an integration test for for list operations.

Cheers! :beers: