Changes

s1ap_proxy: move S1 SETUP handling to sctp_proxy

This is a prerequisite for the MME pooling support.  We need to receive
the S1 SETUP REQUEST PDU, extract the Global-eNB-ID and TAC(s), and use
this information to select an appropriate MME.  The PDU must also be
cached so it can be sent (or re-sent) to the selected MME(s).

The s1ap_proxy module no longer needs to parse S1 SETUP PDUs, as this
logic has been moved to the sctp_proxy and implemented using the newly
introduced s1ap_utils API.  However, s1ap_proxy still requires the
Global-eNB-ID for logging and per-eNB counters, so sctp_proxy now
provides it through the new s1ap_proxy:set_genb_id/2 API.

Change-Id: I9aa67732b418bcdf3f10b2db89a41dda26ee3d4e
Related: SYS#7052