Fix SR multicast post mfib commit

Fix SR multicast post mfib commit
 1 - use the SR policy to construct the replicate DPO. Each bucket therein is a SR tunnel.
 2 - install a special mfib entry that links via this replicate
 3 - forwarding is now mfib-lookup -> replicate -> sr_rewrite (per-tunnel)
        no need for a separate sr_replicate node.
 4 - Stack the sr tunnel on the forwarding DPO of the first-hop FIB entry.
        no need for a second lookup post SR encap.
 5 - fix some path-list lock leaks in the MFIB entry.

Change-Id: I20de96ea4c4be4fae252625bde159d9c435c8315
Signed-off-by: Neale Ranns <nranns@cisco.com>