Allow updates with timestamps to be evicted to lookaside

Testing of internal MongoDB applications has detected situations where MongoDB 4.2.0-rc2 behaves worse than 4.0.x under cache pressure situations.

In particular, in situations where 4.0.x would use the cache overflow mechanism to make progress during slow checkpoints, 4.2.0-rc2 instead appears to waste time doing update/restore eviction even when it does not contribute to reducing the volume of dirty data in cache.