When there is a small change to a large record, WT should be able to write a page delta that only includes the change to the record, rather than (as today) the complete modified record. 

In other words, page deltas should be able to write (and read back) deltas to record values. This might be represented by a WT_MODIFY or a list of them. 

Motivation: Some applications make frequent small changes to large documents. The resulting write amplification becomes a performance bottleneck. WT writes are constrained by the write bandwidth to the underlying storage/log service. If most of that bandwidth is consumed by writing unmodified record data, the rate at which customers can make small changes is similarly constrained, sometimes to quite low levels of throughput.