reduce datausage on reset/wakeup

[m-mcgowan]

Reduces data consumption when the device is reset or wakes up from deep sleep.

Computes a checksum of the last set of subscriptions, functions, variables and system modules that were sent to the cloud, which is maintained in retained storage, as part of the session.

Before sending a hello after resuming the session, the device checks the current checksum for the subscriptions, functions variables and system modules. If it is the same as the persisted checksum, the hello message is stkipped, since the local application state is the same as what was last sent to the cloud. When the checksums are not the same, the hello message is sent, which clears the app state on the server, so all subsequent describe messages and subscriptions are resent.

For this to work optimally, code should use SEMI_AUTOMATIC mode and connect to the cloud after registering all subscriptions, variables and functions.

e.g.

SYSTEM_MODE(SEMI_AUTOMATIC);

void setup()
{
     Particle.function(...); 
     Particle.subscribe(...);
     Particle.variable(...);

    Particle.connect();
}

---

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)

[m-mcgowan]

Test app:

#include "Particle.h"

SYSTEM_MODE(SEMI_AUTOMATIC);
STARTUP(System.enableFeature(FEATURE_RETAINED_MEMORY));

int valueInt = 0;

void handler(String name, String value)
{
}

int increment(String args)
{
    return ++valueInt;
}

void setup()
{
    Particle.subscribe("hello", handler, MY_DEVICES);
    Particle.variable("value", valueInt);
        Particle.function("inc", increment);
    Cellular.on();
    Cellular.resetDataUsage();
    Cellular.connect();
}

void showDataUsage()
{
    CellularData data;
    Cellular.getDataUsage(data);

    Serial.print("data usage:");
    Serial.println(data);
}

void loop()
{
    if (Serial.available())
    {
        char c = Serial.read();
        switch (c)
        {
        case 'C':
            Particle.connect();
            waitUntil(Particle.connected);
            break;
        case 'd':
            showDataUsage();
            break;
        case 'r':
            System.reset();
            break;
        case 's':
            System.sleep(SLEEP_MODE_DEEP, 120, SLEEP_NETWORK_STANDBY);
            break;
        case 'c':
            Particle.disconnect();
            break;
        }
    }
}

Verify data usage reduction test:

1. flash the app to your Electron by compiling in the cloud against 0.5.0 (production)
2. power down fully, then power on. The device will connect and breathe green.
3. connect to serial, and press 'd' and verify data usage is 0. (Only connected to tower.)
4. press 'C' to connect to the cloud, and then 'd' to retrieve the data usage.
5. press 's' to deep sleep the device
6. press reset to wake up the device and reconnect to serial
7. press 'd' , and verify data usage to connect to the tower is 0 (as measured by the device.)
8. press 'C' to connect to the cloud, and then 'd' to retrieve the warm boot data usage.
9. flash system firmware from this PR, and recompile the test app locally
10. repeat these steps and verify the data usage on warm boot is significantly less.

Test results (mdma):

# running against 0.5.0 system firmware
# start from power-down
# connect to cellular data usage: *d*
data usage:31,0,0,0,0
# cold connect to cloud data usage: *C*
data usage:31,2671,2257,2671,2257
# sleep: *s*, reset
# connect to cellular data usage: *d*
data usage:31,0,0,0,0
# warm connect to cloud data usage: *C*
data usage:31,2673,2257,2673,2257

# running against this PR
# start from power-down
# connect to cellular data usage: *d*
data usage:31,0,0,0,0
# cold connect to cloud data usage: *C*
data usage:31,2737,2325,2737,2325
# sleep: *s*, reset
# connect to cellular: *d*
data usage:31,0,0,0,0
# warm connect to cloud data usage: *C*
data usage:31,74,61,74,61

Additional tests:

• flash tinker, verify funcs/vars, flash another app and verify they change

[m-mcgowan]

@technobly - this one's ready for review.