Row search cleanups.

@michaelcahill, some (hopefully) minor row-search cleanups that have been on my list for awhile.

There are a couple of possibly interesting changes (which is why I didn't just merge this branch), I'll call them out separately in the code.
I agree -- I've checked the callers of `__cursor_skip_prev`, and they all have `cbt->ins` set, which implies a non-empty skip list.