Optimizations to improve dataset ensuring performance (#7236)

Major part of #7217

Optimizations made:
- `zfs get` is queried, so properties are samples for all datasets of
interest ahead-of-time. No subsequent processes are exec'd for each
dataset that needs no changes.
- The "dataset ensure" process is now concurrent

These optimizations should significantly improve the latency of the "no
(or few) changes necessary" cases.

Optimizations still left to be made:
- Making blueprint execution concurrent, rather than blocking
one-sled-at-a-time
- Patching the illumos_utils "Zfs::ensure_filesystem" to re-use the
pre-fetched properties, and minimize re-querying

illumos-utils/Cargo.toml

@@ -21,6 +21,7 @@ dropshot.workspace = true
 futures.workspace = true
 http.workspace = true
 ipnetwork.workspace = true
+itertools.workspace = true
 libc.workspace = true
 macaddr.workspace = true
 omicron-common.workspace = true