partially fix #3111: further improve removing replication snapshots

partially fix #3111: further improve removing replication snapshots

by using the new $blocker parameter. No longer remove all replication
snapshots from affected volumes unconditionally, but check first if
all blocking snapshots are replication snapshots. If they are, remove
them and proceed with rollback. If they are not, die without removing
any.

For backwards compatibility, it's still necessary to remove all
replication snapshots if $blockers is not available.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>