Stunprober interface, its implementation and a command line driver.

Chrome will only see stunprober.h and stunprobercontext.h and link with libstunprober.a.

It has support for shared and non-shared mode. In shared mode, a socket will be used to ping all resolved IPs once. In non-shared mode, each ping will get a new socket.

The thread scheduling will try to run MaybeScheduleStunRequest every 1 ms. When the time is up for next ping, it'll send it out.

BUG=4576
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/51729004

Cr-Commit-Position: refs/heads/master@{#9194}