Sanitize link_manager interface, correct in FlexRAN/PROTO_AGENT

This commit cleans up the link_manager interface:
* don't provide peer_addr/port in create_link_manager() (for TCP/SCTP it is not
  needed, so in the UDP case in needs to be set explicitly)
* Make FlexRAN connect again (correct hardcoded address)
* Fix FlexRAN: retain the manager pointer
* Fix FlexRAN: store receive_queue pointer
* the link_manager's sender thread passes the correct peer_addr/port for the
  UDP case