When destroying a dataset it is necessary to remove all snapshots,
even those not part of the blockstore subsystem. This adds a path
to allow that.

If we destroy the Blockstore DB state first and then the server-side object
destruction fails, we leave behind a dangling object and potentially Lease DB
state. But if we destroy the object first and the DB state removal fails,
we are left with a blockstore with no server-side object and thus we cannot
look it up in the future to retry the destruction.

We choose to go with the latter and just create a tiny server-side stub
object if the DB state removal fails.

You can now simultaneously RW and RO map a dataset because all the RO
mappings use copies (clones) of a snapshot. Only a single RW mapping
of course.

When the RW mapping swaps out it automatically creates a new snapshot.
So there is currently no user control over when a version of the dataset
is "published", it just happens everytime you swapout an experiment with
a RW mapping.

A new RW mapping does not affect current RO mappings of course as they
continue to use whatever snapshot they were created with. New RO mappings
with get the most recent snapshot, which we currently track in the DB via
the per-lease attribute "last_snapshot".

You can also now declare a lease to be "exclusive use" by setting the
"exclusive_use" lease attribute (via modlease). This means that it follows
the old semantics of only one mapping at a time, whether it be RO or RW.
This is an alternative to the "simultaneous_ro_datasets" sitevar which
enforces the old behavior globally. Primarily, I put this attribute in to
prevent an unexpected failure in the snapshot/clone path from wreaking
havoc over time. I don't know if there is any value in exposing this to
the user.

The -f <fstype> option to createdataset will now pre-initialize the dataset
with an empty filesystem. Supported types are: ufs and ext[234].

Needed in order for non-admin lease commands to work.

This specifies what types of bstores are allowed to be allocated from
the space. Currently, just used to isolate datasets to one particular pool.