Incremental backup

@michaelcahill  I still have a test or two to write, but here is the basic code and a unit test.
As far as I can see, we only ever check that the URI starts with "log:".  We should make sure nobody calls truncate with `target=("log:foo")` -- I have no idea what they would want that to do...  In other words, I think there should be at least one `WT_STRING_MATCH` somewhere.