Any number of users/experiments can mount a given dataset (given that
they have permission) in read-only mode.  Attempts to mount RW will
fail if the dataset is currently in use.  Attempts to mount RO while
the dataset is in use RW are also prohibited.

Under the hood, iSCSI lease exports (targets) are now managed per-lease
instead of per-experiment.  The set of authorized initiators (based
on network) is manipulated as consumers come and go.  When the last
consumer goes, the export is torn down. Likewise, if there are no
current consumers, a new consumer will cause an iSCSI export to be
created for the lease.

Also included in this commit is a small tweak to implicit lease permissions.