enb_proxy: add initial MME pooling support

Rework the CONNECTING state to dynamically select an MME from the pool
via mme_registry:mme_select/1, passing the eNB's Tracking Area Codes
(from the ?'id-SupportedTAs' IE of the S1 SETUP REQUEST) and a list of
already-tried MMEs, so successive attempts pick a different candidate.

On connection failure (SCTP establishment timeout or error), or when the
selected MME rejects the S1 SETUP REQUEST or fails to respond in time,
the FSM re-enters the CONNECTING state rather than terminating.  This
triggers another mme_select/1 call with the failed MME added to the
tried_mmes list.  S1 SETUP FAILURE PDUs from the MME are intentionally
not forwarded to the eNB, so the retry is fully transparent.

Once mme_select/1 exhausts all candidates it returns 'error'; at that
point the FSM builds and sends an S1 SETUP FAILURE PDU to the eNB and
terminates.

Other changes:
* add close_sock/1, close_conn/1 helpers; simplify terminate/3
* add ?S1GW_CTR_ENB_PROXY_MME_SELECT_ERROR counter

Change-Id: I83dc4a78c78a7b87e87f5ca9a941a168d6c1dc36
Related: SYS#7052
---
M include/s1gw_metrics.hrl
M src/enb_proxy.erl
M src/s1ap_utils.erl
M src/s1gw_metrics.erl
4 files changed, 84 insertions(+), 33 deletions(-)