IoT Edge device storage used for storing pending messages doesn't clear even if messages have already been sent by edgeHub module

## Summary
IoT Edge device storage used for storing pending messages doesn't clear even if messages have already been sent by **edgeHub** module upstream to Azure IoT Hub. Files holding the storage messages should be **deleted** and storage needs to be constantly reclaimed as messages are sent to IoT Hub. This will also help user to track if there any pending messages still left to be sent.

## Steps to Reproduce
Provide a detailed set of steps to reproduce the bug.
1.) Set the storage folders for offline mode. Link module storage to device storage

Below links cover this in detail
https://docs.microsoft.com/en-us/azure/iot-edge/offline-capabilities?view=iotedge-2018-06
https://docs.microsoft.com/en-us/azure/iot-edge/how-to-access-host-storage-from-module?view=iotedge-2018-06

2.) Run the simulated temperature sensor module. Set it to send message every **1 second**. Set unlimited message count (-1)
3.) After **10 minutes** or so temporarily block access to IoT Hub IP address.
4.) Message will start accumulating on edgeHub. edgeHub will also start throwing connectivity exception as it goes into offline mode.
5.) After **30 minutes**, unblock the IP address of IoT Hub.

## Expected Behavior
1.) All pending accumulated messages during offline mode are sent to IoT Hub
2.) Files used to store pending messages are cleared after 1 hour or so. Storage **shouldn’t** continuously grow.

## Current Behavior
1.) All pending messages go through when edgeHub connection to IoT Hub is restored
2.) Storage continues to grow. Files used to store pending messages are not cleared. There is no flush even after **1 hour**

## Context (Environment)
Ubuntu 18.04.2 LTS

### Device Information
* Host OS : Ubuntu 18.04.2 LTS
* Architecture: x86_64
* Container OS Linux containers

### Runtime Versions
* iotedged [run `iotedge version`]: : iotedge 1.0.10

* Edge Agent [image tag (e.g. 1.0.0)]:
* Edge Hub [image tag (e.g. 1.0.0)]:
* Docker/Moby [run `docker version`]:

Note: when using Windows containers on Windows, run `docker -H npipe:////./pipe/iotedge_moby_engine version` instead

## Logs

iotedged logs

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edge-agent logs

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edge-hub logs

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## Additional Information
Please provide any additional information that may be helpful in understanding the issue.