Split.

Experimental branch, looking at doing splits differently.

@agorrod, @michaelcahill: this branch isn't ready for review, I'm pushing it just to make it easier to build/run on different machines.   (Feel free to review of course, all comments welcome, but I'm not asking for review yet.)

The state is I can split into parent pages as part of eviction, but I cannot deepen the tree permanently, that is, I haven't tried to write the code to split internal pages that grow too large.  This tree mostly runs and passes tests, but I busted the overflow tracking code somewhere, so test/format runs fail if there are significant numbers of overflow items.   I also haven't run anything other than single-threaded tests, yet.