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.
Looking at it, yes, we should be able to do so.  The only concern is the `!file:` piece of that conditional.  What other dhandles could we have that are metadata-like that we may miss?  That was added over 2 years ago, is it possible it is outdated now and just the metadata handle is sufficient, and in fact, the only thing we'll find?  I'll get rid of this loop.