Add incremental backup performance focused test

In addition to our correctness testing, we should add testing for incremental backup and performance. This could be a variant or part of test/csuite/incr_backup or it could be its own new standalone test.

The test should create several/many files that are fairly large and compare the amount of time it takes to do a full backup against an incremental. It likely wants to use a smaller granularity in order to force a lot of bits to be needed. It should compare times while modifying a hot key (or small number of keys) with a full backup. With a hot key the incremental should be much faster.