Changes

s1ap_proxy: turn this module into a gen_server

This patch makes the s1ap_proxy module self-contained, so API users
no longer need to initialize, maintain, or deinitialize its internal
state, and also no longer need to trap 'EXIT' signals from the child
erab_fsm processes.  These improvements in maintainability and ease
of use come at the cost of an additional process per SCTP
connection, which is manageable.

Change-Id: Id6e4c7ee29ae31edca658e1293601d38e5f43e63