datasets in ramdisk pool want primarycache=none

[wesolows]

This data is already present in pinned RAM. There is nowhere to cache it that's faster; doing so is a waste of memory. These datasets should be created with primarycache=none. Any runtime additions to illumos that create more datasets in this pool must also be written to do so.

These datasets are small, so the waste is relatively modest; therefore I'm tentatively assigning this to MVP+1 as we can likely survive without it at some performance penalty.

[citrus-it]

This has been integrated as part of #89