Now that MongoDB is enabling timestamp usage, when there is replica lag WiredTiger needs to retain more history in cache than it did before. This work makes our cache overflow mechanism more efficient to handle that situation as gracefully as possible.

The changes in WT-3652 helped many workloads run without interruption when lookaside eviction is required. However, in the course of that work, some performance issues surfaced under heavy insert workloads, and we should be able to improve the behavior and performance of the lookaside table for those cases.