CON-2328: DataMartService: add a fallback when replication does not work

Hide ``getTopArticlesByPageview`` method behind a helper that performs a fallback to the last known rollup

A Nasty And Ugly Hack (TM) before we have a proper rollups solution.

@see https://wikia-inc.atlassian.net/browse/OPS-5465

@kvas-damian / @SebastianMarzjan / @drozdo 
incorrect indentation