Harden trigger and callback registrations in CacheEntry

[Tratcher]

There are possible race conditions in CacheEntry between AttachTriggers and DetatchTriggers. Similar for the callback APIs. These need to be hardened to prevent potential stress/race crashes.

[BrennanConroy]

Every call to InvokeEvictionCallbacks is wrapped in the ReaderWriterLock in MemoryCache so it looks impossible to invoke more than once

[BrennanConroy]

#126 and #132