Add locking to CacheServiceMonitor

[I3undy]

@Piedone created on Aug 12, 2015:
https://helpfullibraries.codeplex.com/workitem/7

The factory delegate in CacheServiceMonitor.GetKeys() can run multiple times on simultaneous requests, causing a stale keychain. It should instead use a distributed lock once its built into Orchard.

[I3undy]

@Piedone commented on Aug 12, 2015:

Also see: OrchardCMS/Orchard#5632

[Piedone]

We won't do this anymore.