Make it explicit that LSM cursors cannot read from checkpoints, at least
until we make that work properly.
Note that WT_CURSOR.insert doesn't hold a position in the object, so
it's to be preferred over WT_CURSOR.update for that reason.
Split the flag asserts so it's easy to know which one fired.
Fix a bug where pages that split and had to be restored while a checkpoint is in progress were not written in that checkpoint.
Merge branch 'develop' into checkpoint-optimizations
Merge branch 'checkpoint-optimizations' into checkpoint-test