Double the time between heartbeats to 20-30 seconds
Double the time between heartbeats to 20-30 seconds

Summary:
- Heartbeats are nearly useless already and are maybe part of the
  cause of increased load on Changes.

Test Plan: staging

Reviewers: paulruan, anupc, kylec

Reviewed By: anupc, kylec

Subscribers: changesbot, kylec

Differential Revision: https://tails.corp.dropbox.com/D186767

dropbox/changes-client,dropbox/changes-client