Improve the heartbeat mechanism.

Summary:
This revision changes the Redis database from 0 to 1 (see
/etc/inboxapp/config.json) - NOTE: we can use any database from 1 to 15;
introduces support for the heartbeat mechanism for EAS; improves both the
location and the parameters of calls to publish() to improve reporting; avoids
unnecessary branches when retrieving a Redis client; improves error handling
with better logging.

In addition, this revision overhauls the bin/monitor-syncstatus script, which
is now bin/inbox-syncstatus. The script now prints JSON on stdout and exits
with either 0, 1, or 2, which makes it play nicely with our sensu-based
monitoring.

Test Plan:
```
vagrant up
vagrant ssh
cd /vagrant
bin/inbox-start

vagrant up
vagrant ssh
cd /vagrant
bin/inbox-auth ADDRESS

vagrant up
vagrant ssh
cd /vagrant
bin/inbox-syncstatus --redis-hostname='localhost' --redis-port=6379 --redis-database=1
```
Happy monitoring :)

Reviewers: emfree, kav-ya

Reviewed By: emfree, kav-ya

Subscribers: khamidou, jennie, spang

Differential Revision: https://review.inboxapp.com/D771