Minor bug fixes in eviction.

@michaelcahill, here's the branch with the 3 changes we discussed.

There's another change in there as well: in `__evict_walk` we used an integer counter to start each eviction file handle traversal immediately after the handle where we ended the last walk, but if we're sweeping handles, that's not going to work any more.  I changed it to use an address in the WT_DATA_HANDLE structure instead, I think it's less work and probably more reliable.