Maintain per ggsn pdp ctx list
This way we can easily track all pdp context associated to a specific
ggsn, which is useful to handle some scenarios, such as the one
implemented in next commit, in which specs references that GSNs should
ping only other GSNs with at least one pdp ctx in common. So the list
of pdp ctx per GGSN is really useful too (and cheap computationally)
to check if we should arm or disarm the echo procedure timer.
So this commit can be seen as a preparation for next commit.
Change-Id: I3bbcc0883df2bf1290ba8d4bd70db8baa494087a