Allow xSPDistributedCacheService to change the service account

Currently xSPDistributedCacheService will set the service account on the cache when it is provisioned, however it will not update the account if the configuration is updated.

@harbars has a great post that has some great scripts and process on this which should be considered when implementing this change.

There are a bunch of things to consider in this though (some taken from the above article):

• we can't change the account when more than one server, so need a process to shut it down from other servers before changing it
• we need to be aware of what will happen if this resource runs on multiple servers at the same time (our ServerProvisionOrder property may prove useful here in that we need to make sure that its right on the first server before attempting to provision on a second? Need to look in to this more)
• We need to be sure the cache size configured here sticks
• SharePoint 2016 will let us change the service account before a server is provisioned, which might provide us an option to implement logic that will do that for 2016 - need to investigate the complexity of this

There are a bunch of considerations here, but this is something we need to get in to xSharePoint at some point to improve on the distributed cache resource and ease some pain of making these changes.

Start by reviewing the xSPDistributedCacheService entry point and the linked Harbar post for the service-account change process. Investigate multi-server coordination, ServerProvisionOrder, cache-size preservation, and SharePoint 2016 behavior. Done means configuration updates can change the account safely across the described scenarios.