Fix contrail-collector back pressure mechanism
Fix contrail-collector back pressure mechanism

contrail-collector SM queue back presssure mechanism was not
working since the SM drop level is initialized to INVALID and
even the water marks levels are INVALID and hence the defer/undefer
callbacks are not called.

Change-Id: Ieaf65d3d77356b9ac471e9479a1fb743c16fe6a1
Closes-Bug: #1528380
(cherry picked from commit 68c2ef7d6b8e6adc36f6003e0bcb97bab310750d)

Juniper/contrail-sandesh,Juniper/contrail-sandesh,Juniper/contrail-sandesh,Juniper/contrail-sandesh