Sync time and unsubscribe called on the system thread

[monkbroc]

Fixes #904

Calling Particle.syncTime() in a multithreaded app has a small chance of crashing the system if the system thread is currently calling network functions too.

The fix is to call Particle.syncTime() on the system thread.

App to reproduce:

#include "application.h"

SYSTEM_MODE(SEMI_AUTOMATIC);
SYSTEM_THREAD(ENABLED);

void setup()
{
  Serial.blockOnOverrun(true); //don't ever block for serial integrity
  Serial.begin(9600);

  WiFi.on();
  WiFi.connect();
  Particle.connect();

  Serial.println("breatheGreen5");
}

static unsigned long millisOfLastPublish = 0;
static unsigned long millisOfLastTimeSync = 0;

void loop()
{
  Particle.process();

  if( Particle.connected() )
  {
    if( ( millis() - millisOfLastPublish ) > (unsigned long)random(260, 500) )
    {
      millisOfLastPublish = millis();
      Serial.println("Publishing event");
      Particle.publish("event", " fjksdas " + String(millisOfLastPublish) );
    }

    if( ( millis() - millisOfLastTimeSync ) > 1 )
    {
      Serial.println("timeSync: " + String( Particle.syncTime() ) );
      millisOfLastTimeSync = millis(); 
    }
  }
}

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Doneness:

• Contributor has signed CLA
• Problem and Solution clearly stated
• Code peer reviewed
• API tests compiled
• Run unit/integration/application tests on device
• Add documentation
• Add to CHANGELOG.md after merging (add links to docs and issues)
  Prevents issues when multiple threads try to send messages through the cloud connection or manage the network state shared memory.