SegmentMk IT tests are too intensive

Seen on the 1.0 branch only so far when running ITs on my local machine, but travis reports the same:

https://travis-ci.org/apache/jackrabbit-oak/builds/51589769

It doesn't necessarily mean the problem is with SegmentReferenceLimitTestIT even though the heap dump shows most of the memory consumed by Segments and SegmentWriter. A recent build on trunk was successful for me where we have the same test.