Change eviction to start new walks from a random place in the tree

The eviction server currently traverses a btree from the beginning or end when it starts a new walk point. For trees that have a very large amount of content in cache cache it is possible that the walk point is always reset before part of the tree is visited.

Our eviction algorithm would be more robust if a new walk point is selected as a random point in the file rather than the beginning or the end.