Remove operation threading from broadcast actions
Similar to search removal, the operation threading options are not really ued, and the default should always be used. This also considerably simplifies the code.
A side affect is that we can now remove the ShardIterator#firstOrNull method, which can cause for sneaky bugs to occur.
closes #6044
[REST] Missing scroll id now returns 404

A bad/non-existing scroll ID used to return a 200, however a 404 might be more useful.
Also, this PR returns the right Exception (SearchContextMissingException) in the Java API.

Additionally: Added StatusToXContent interface and RestStatusToXContentListener listener, so
the appropriate RestStatus can be returned

Closes #5729