ENDC for 5G NSA support (preparatory work): Basic implementation of x2ap_eNB_generate_ENDC_x2_setup_request() and x2ap_gNB_handle_ENDC_x2_setup_request()

Dual connectivity: Basic implementation of x2ap_eNB_handle_senb_addition_request and x2ap_eNB_generate_senb_addition_request_ack

- basic version of x2ap_eNB_generate_senb_addition_request with dumb values
- skeleton functions:
    - x2ap_eNB_handle_senb_addition_request
    - x2ap_eNB_handle_senb_addition_request_acknowledge
    - x2ap_eNB_handle_senb_addition_request_reject

The 5G phone OPPO has a very big UE capability (> 2KB).

We need bigger buffers to transfer this, especially in x2 handover.

Also the sctp messages need a bigger buffer.

Note: in sctp, we didn't check for partial messages, it's now
      done. When detected, we exit. To be redone differently
      if needed. (Look for "if (!(flags & MSG_EOR))".)

To be tested: Add code for measurement report based on the neighbouring cells after x2 setup, ocs/ocn for multiple cells

ReFix Coverity Scan CID 339937 (Overrunning array x2ap_messages_callback of 27 24-byte elements at element index 27 (byte offset 648) using index pdu.choice.successfulOutcome.procedureCode (which evaluates to 27).)

This commit introduces X2AP timers (t_reloc_prep, tx2_reloc_overall).
You need to set the values in the configuration file.

X2AP can be enabled or disabled in the configuration file too (disabled
by default).

Some deadcode was removed.

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

Some deadcode was removed.

The implementation of the ID management is not finished.
It will only work if the handover is successful. No error
recovery has been done. For error recovery, we need to
implement the X2AP timers. As it is, in case of error, an ID
might very well never be released.

It has been tested with 1 and 2 UEs doing handover more or less
at the same time.

- manage target eNB based on target cell id received by measurement report
  for Handover Request
- manage source eNB based on association id for Handover Request Ack
- use only x2ap instance and not x2ap data instance points to x2ap instance

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

S1AP PATH SWITCH REQUEST/REQUEST ACK/FAILURE
RRC MEASUREMENT REPORT
MINOR CHANGES IN MAC LAYER

SCTP: one to many sockets implementation