Split generation with safe

This branch has a fix for WT-1799 (8bdb29f) and a fix for a bug (found by inspection), where LSM calls the __wt_btree_size_overflow function without setting its split-generation (1b1371c).

There are also general changes to the macros that look at internal page WT_PAGE_INDEX values: the idea is any code acquiring a WT_PAGE_INDEX reference either uses a macro that makes it clear we believe it's "safe", or we assert the split-generation is set.

@agorrod, @michaelcahill, would one of you please review?