Revert "Only lock when replicating."
Revert "Only lock when replicating."

Testing have revealed that destroying snapshots while used
in replication fails because the dataset is in use.
Reintroduce global locking. Per file system is probably the
best option.

This reverts commit 9176e82fe8f23e416deb1deef61b1e63f15b6203.

hkbakke/zfssnap,hkbakke/zfssnap