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.
It is to match the conditional in `txn/txn_ckpt.c`:420 (of this branch).  The checkpoint code is checkpointing the metadata and is explicitly looking for `WT_IS_METADATA` and `!file:`.  I wasn't sure what circumstances get us to have something `!file:` that is not just the metadata file, but it has to be on the same bucket list (unless we optimize it out based on Michael's suggestion).  So that motivated this change.