Rework federation merge functions to utilise advantages of Streams

[tb06904]

Currently the merge functions utilised by the federated store don't use any of the Java Stream classes. These could bring performance enhancements due cheaper per element cost to spliterators or simplify code with methods like concat(), flatMap() and reduce().

Proposal is to refactor/rework these functions to take advantage of the Stream library where applicable. As an example you can merge two streams like:

Stream<Object> currentStream = StreamSupport.stream(current.spliterator(), false);
Stream<Object> updateStream = StreamSupport.stream(update.spliterator(), false);
// This cast satisfies the Iterable interface requirement to support an arbitrary number of iterations
Iterable<Object> merged = () -> Stream.concat(currentStream, updateStream).iterator();
return merged;

Originally posted by @GCHQDeveloper314 in #3072 (comment)