zone logs should not be in rpool #3533
Description
Currently, our zones are created from the RAM disk zpool (rpool). A challenge with that in particular is that zone logs are now both volatile (a challenge for debugging if a sled is bounced) and -- more pressingly -- very tightly capped. In particular we had an instance where a pantry log was 21.6G (a feat only made possible by the ~10X compression factor we were getting there). Once things right out of space, the world gets grim in a hurry. We want to trim logs and log less loquaciously and so on -- but we also want/need a bit more breathing room by getting zones either onto one of the oxi_ (M.2) pools or one of the oxp_ (U.2) pools. Because we saw this in dogfood, marking this MVP.