Use dhandle hash lists in more places.  Adjust sweep timings. #1460

@agorrod Please review this set of changes.  I implemented a few of the dhandle list walk optimizations that I mentioned in #1460.  I also took a compromise position on the sweep timings, setting them to 30 seconds TOD and 10 second sweep period.
Can we get rid of this loop by adding `__wt_metadata_open` at the top of `__wt_txn_checkpoint`, and just using `session->metafile` directly here?