Added isStarted property to TrueTimeClient and NTPClient.

[AlexMokrushin]

What did you change and why?

Added isStarted property to TrueTimeClient and NTPClient.
If start() is called occasionally twice, app crashes on check :

precondition(self.reachability.callback == nil, "Already started")

We can avoid this crash calling start() like that:

let client = TrueTimeClient.sharedInstance
if (!client.isStarted) {
    client.start(pool: [NTP_HOST], port: NTP_PORT)
}

Potential risks introduced?

No new risks.

What tests were performed (include steps)?

Manual tests.

Checklist

• Unit/UI tests have been written (if necessary)
• Manually tested