Avoid clearing the saved last-key when no instantiated key

Performance regression got introduced in 4.2 version compared to 4.0 when the test contains 5 collections with 10 indexes each. Indexes are designed to have keys with substantial common prefixes; this seemed to be important to generate the issue. Collections are populated, then are sparsely updated in parallel, aiming for roughly 1 update per page, in order to generate dirty pages at a high rate with little application work.

More details about the discussion of the issue can be found in SERVER-44991.