Fix a comment, modifications more recent than the checkpoint are no longer
a problem for eviction, but checkpoints are still a problem.
Put the checkpointing tests together in one place.
Create another union in WT_PAGE_MODIFY, for internal vs. leaf pages,
gets the size of WT_PAGE_MODIFY down to 96B.
Move the split functions into rec_split.c (removing the previous
contents), remove rec_merge.c entirely.