Add support for expiring remote snapshots

[gcooper]

The code in autosnap.py handles ZFS snapshot expiration on the local machine:

for snapshot in snapshots_pending_delete:
    zfsproc = pipeopen('/sbin/zfs get -H freenas:state %s' % (snapshot))
    output = zfsproc.communicate()[0]
    if output != '':
        fsname, attrname, value, source = output.split('\n')[0].split('\t')
        if value == '-':
            snapcmd = '/sbin/zfs destroy %s' % (snapshot)
            system(snapcmd)

A user requested that we add replicated snapshot expiration support, so the remote end doesn't fill up the remote filesystem with replicated snapshots, s.t. manual intervention would be required to fix the issue.

[william]

Duplicated of #388