Functionality to track "first run" in cache objects

[StanleyGoldman]

As mentioned in #409

We want to start using the Invalidated events on cache objects to know when we need to re-populate data.

For some of those cache objects, we want to fire an implicit invalidated on the application's "First Run" as opposed to a domain reload. That way we know these objects need to be populated immediately.

This change adds:

• functionality to track the first run time in ApplicationCache
• functionality to track when ManagedCacheBase objects are initialized
• functionality in ManagedCacheBase.ValidateData() to verify if cache objects were initialized after application first run time, otherwise it will update and fire an "Invalidated" event
• functionality to define which cache objects are invalid on first tun

This provides base functionality to:

• Utilizing a cache for user data #427
• Repository using cache invalidation events to update data #436

Depends on:

• Removing unused TaskManager from RepositoryManager to fix compiler warning #435

[jcansdale]

I tried to leave an inline-comment, but was denied. I'll paste it here instead:

[shana]

All of these DateTimeOffset.ToString() calls need to specify the ISO 8016 format, eg. 2017-11-16T20:24:46+00:00. As it is, they're outputting strings in whatever local configuration the system has, which may not be parseable by Parse.

[StanleyGoldman]

Originally fixed in #457 carried over here

[StanleyGoldman]

@jcansdale Unity cannot serialize DateTimeOffset objects, but it will serialize string objects. So for all these dates we have a string field and a DateTimeOffset? field; the latter of which we populate lazily. I did however make a change based on your comment, there was no reason to set the lazily generated value to null and thus wait for the lazy generation.

[drguthals]

Seems straight forward and like a clean approach.