DO NOT MERGE: Ignore unreliable DNS servers.

Collect statistics about DNS query success state and delay. Ignore
servers that have been tried at least five times and have a success rate
of < 0.25. Retry these servers once every 180s.

Bug: 25731675

(cherry picked from commit 6b3f0d65)

Change-Id: Id989404b14fd885fb31a5a4de36a4186be1c9ad1