agent crash upon upgrade from 2.20-64 to 2.20.x-3 with snat instances

[Senthilnathan Murugappan]

Observed below agent crash on couple of nodes during upgrade from 2.20-64 to 2.20.1-3
The system had 20 service instances and the nodes had 9 and 6 netns instances

(gdb) bt
#0 0x0000000000ea3150 in DBGraphVertex::adjacency_iterator::adjacency_iterator(DBGraph*, void*) ()
#1 0x000000000099e47f in ServiceInstance::CalculateProperties(DBGraph*, ServiceInstance::Properties*) ()
#2 0x000000000099f4e4 in ServiceInstanceTable::OnChange(DBEntry*, DBRequest const*) ()
#3 0x0000000000ea5a3d in DBTable::Input(DBTablePartition*, DBClient*, DBRequest*) ()
#4 0x0000000000ea5226 in DBPartition::QueueRunner::Run() ()
#5 0x0000000000fa2f20 in TaskImpl::execute() ()
#6 0x00007f0bb61f0b3a in ?? () from /usr/lib/libtbb.so.2
#7 0x00007f0bb61ec816 in ?? () from /usr/lib/libtbb.so.2
#8 0x00007f0bb61ebf4b in ?? () from /usr/lib/libtbb.so.2
#9 0x00007f0bb61e80ff in ?? () from /usr/lib/libtbb.so.2
#10 0x00007f0bb61e82f9 in ?? () from /usr/lib/libtbb.so.2
#11 0x00007f0bb640c182 in start_thread (arg=0x7f0b97fff700) at pthread_create.c:312
#12 0x00007f0bb56e547d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

The core is at 10.84.5.31:/cs-shared/bugs/<bug-id>

[Divakar Dharanalakota (ddivakar)]

https://review.opencontrail.org/14249
is the cherry-pick from R2.20 to resolve this issue.
-Divakar