RavenDB-1554 More efficient prefetcher work under heavy update scenario
Merge branch 'RavenDB-1554-another-attempt' of https://github.com/arekais/ravendb
Moving implementation into a single class.
Avoiding unnecessary allocations.
Fix to support importing or exporting multiple document types via Smuggler

Only one document type could be specified via smuggler.
Now supports syntax like: --metadata-filter=Raven-Entity-Name=Posts,Persons,Organizations
Do not consider empty items as distinct
Merge branch 'SmuggleDocs' of https://github.com/steinho/ravendb
Better logs
Making sure that we will register the work being done when we have an outgoing stream still active