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.
It seems like we were overloading the `appending` flag to mean two different things: go to the last page in the file (when searching internal pages) and check for appends to skip lists.

What you're doing here is definitely better -- maybe we could consider two different flags (one for internal pages, the other for skip lists)?  Maybe also consider moving these flags into `WT_CURSOR_BTREE` so that each cursor is optimised for its own pattern of accesses, rather than trying to guess across a whole tree.

I'll merge -- I'm curious to see what happens with some of our workloads now that we aren't fast-pathing appends to skip lists  on all pages: random inserts should get faster by avoiding a comparison, but some other patterns of inserts will end up doing more searches.