Add draft infrastructure to monitor the sync status.

Summary:
This commit introduces a draft infrastructure to monitor the sync
status. When an account monitor starts a new folder engine, the folder
engine ctor instantiates an object of type SyncStatus and eagerly signal
its presence to the world by publishing a timestamp on Redis with a key
composed of the account and folder identifier (this makes searches for
all folders of a given account really simple with a "regex" like
"(<account idenfitier>, *)").
As of now, folder engines keeps signaling their "state" every 60 seconds
(this is a configurable parameter at SyncStatus instantiation) and
eagerly do so when the "state" changes (e.g., on startup or state
transition).

NOTE: reducing the interval between subsequent signal operations (e.g.,
from 60 to 10 seconds) may not work correctly since the folder engines
are sleeping for "poll_frequency" and might also idle. This should not
be a huge limitation since having up to date monitoring within a minute
should be totally reasonable.

WARNING: This commit also add a very primitive script to flush ALL the
keys of ALL the dbs handled by the Redis instance pointed by the
REDIS_HOSTNAME, REDIS_PORT contained in /etc/inboxapp/config.json.

Add Redis configuration parameters to etc/config-test.json.

NOTE: this revision also revert a few commits I made on when I was "playing"
with sync status.

Test Plan:
Nothing really. You can see very basic information by accessing Redis from its
CLI. Try redis-cli from within the virtual machine and then scan 0 while
accounts are sycing.

Reviewers: kav-ya, emfree

Reviewed By: emfree

Subscribers: spang

Differential Revision: https://review.inboxapp.com/D746