RavenDB-2250 fix -> tweaks for better handling of large docs during indexing

summary:
* auto tuner will hold information about document sizes in currently running document batches --> this information is now also used to determine whether to make batch smaller or not
* indexing and reduce executer "report" when they begin and end indexing --> this maintains the list of sizes in auto tuner
* prefetcher has additional checks when adding future batches to not go over the memory limits