Small cleanup comments, performance
micro optimization to DictionaryVolatileCache cacheEntry mgmt.
removed unused private class.
Added ILongTermCacheEntry.cs and DefaultLongTermCacheEntry.cs
Converted ILongTermCache to use ILongTermCacheEntry.cs
Change both get and set.
Implemented the expiration policy for LongTermCacheLookup.
Tests to support:

Converted ILongTermCache to use ILongTermCacheEntry.cs
Change both get and set.
Implemented the expiration policy for LongTermCacheLookup.