feat: add new strategy to route requests through NATs

Overview

This MR aims at implementing a new strategy to route requests when UACs route requests through NATs. The implemented feature is described in the following RFC: RFC5626, sections 5.1 to 5.3.

This MR maintains compatibility with the current method (called "contact-correction") and adds a new method called "flow-token".

WARNING: at the moment, this new method is not intended to work when updating a session.

FLEXISIP-55

TODO

• Fix tests so they do not use port 5060
• Generate a random hash key and store it under /var/lib/flexisip
• Write unit tests
  • Flow
  • FlowUtils
  • FlowFactory
  • SocketAddress
  • NatHelper configuration parameters
  • ContactCorrection strategy
  • FlowToken stategy
• Write integration tests (feature-level tests)
• Update module NatHelper description in configuration file
• Write doc strings
• Problem when pausing a call between pc and mobile --> udp