Add niceAgentIceTcp configuration option and API methods for it

[prlanzarin]

What is the current behavior you want to change?

Currently, ice-tcp is not explicitly set in Kurento's NiceAgent usage; therefore, it defaults to TRUE (libnice's default) and always does TCP candidate gathering.

Property reference: https://libnice.freedesktop.org/libnice/NiceAgent.html#NiceAgent--ice-tcp

What is the new behavior provided by this change?

The rationale behind this is to give the ability to deactivate TCP candidate gathering in setups where they aren`t needed, thus potentially speeding up the gathering process/connectivity checks and trimming down the number of generated candidates/bound sockets.

From the WebRtcEndpoint.conf.ini:

;; Whether to enable TCP candidate gathering via NiceAgent's ice-tcp property.
;; The motivation behind this is to potentially speed up gathering
;; by preventing TCP candidate gathering in scenarios where they aren't needed.
;; Property reference: https://libnice.freedesktop.org/libnice/NiceAgent.html#NiceAgent--ice-tcp
;;
;; <niceAgentIceTcp> MUST be either 0 (FALSE) or 1 (TRUE)
;; Default is 1 (TRUE)
;niceAgentIceTcp=1

A pair of API methods was also added to be able to control this per element:

• WebRtcEndpoint.setNiceAgentIceTcp(true|false)
• WebRtcEndpoint.getNiceAgentIceTcp

How has this been tested?

• Compiled on 16.04
• Manually tested:
  • WebRtcEndpoint.conf.ini#niceAgentIceTcp: 1|0, by observing about:webrtc and chrome:webrtc-internals dumps to verify the generated server-side candidates
  • WebRtcEndpoint#setNiceAgentIceTcp(true|false), via the JS client, by observing about:webrtc and chrome:webrtc-internals dumps to verify the generated server-side candidates
• Pending: unit tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature / enhancement (non-breaking change which improves the project)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• My change requires a change to the documentation
• My change requires a change in other repository

Checklist

• I have read the Contribution Guidelines
• I have added an explanation of what the changes do and why they should be included
• I have written new tests for the changes, as applicable, and have successfully run them locally

[jenkinskurento]

Hi there, thanks for your Pull Request!

A Kurento member needs to verify that this patch is reasonable to test. In case it is, they should write a comment with the phrase test this please. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by Kurento members will still work. Regular contributors can be whitelisted to skip this step.