ggsn: Implement echo req/resp and recovery

This patch is quite big because implementing echo req/resp and recovery
requires having knowledge and managing differentiated state for each GSN
peer attached/connected to osmo-ggsn. This kind of information was not
available in osmo-ggsn nor in libgtp.

So osmo-ggsn is now able to track GSN peers connected to a
ggsn_ctx (associated gsn_t from libgtp) by means of "sgsn_peer" data
structure, and accessible from the ggsn through a list. The instances of
sgsn_peer are currently allocated and destroyed dynamically based on
discovered peer who have at least a pdp context attached to us (we are
not interested in peers without pdp contexts because we don't need to
send echo requests/responses and maintain state in that case).

A new private pointer (pdp_t->priv) data structure struct pdp_priv_t is
added to be able to relate a pdp_t to an sgsn as well as the already
existing pointer to an apn.

An "echo-interval <0-36000>" VTY command is added which allows
configuring time wait between echo requests being sent to each
sgsn_peer. Transmission of echo requests is disabled by default.

Finally, a new "show sgsn" VTY command is introduced, and its output is
also printed during "show ggsn".

Related: OS#4165
Change-Id: Id2c84165dc59dff495106758146a701ca488834f

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