Sled Agent destroys crypt/zone filesystems if it hasn't seen them before

[smklein]

This builds on #3638 , with a slightly more fine-tuned mechanism.

• crypt/zone is created on new U.2s, if it doesn't already exist
• If it does exist, then Sled Agent needs to make a call: Should I delete the dataset, possibly containing zones from last boot, or should I leave it alone (for idempotency)?
• In Only wipe crypt/zone if this is the first time we're mounting it #3638 , we used "was the filesystem previously mounted" as a proxy for this information. However, in cases where the sled agent reboots, this information gets out-of-date -- the filesystem could be mounted, but sled agent should remove the corresponding dataset.

This PR introduces a filesystem property, simply called 'oxide:agent', which is a randomly generated string created statically during the lifetime of the Sled Agent. If the sled agent reboots, this value will be regenerated, which instructs the sled agent to wipe crypt/zone and re-create it.