Improvements on getCacheEntry() synchronization

• Separate GetKeyValueCommand into two, GetKeyValueCommand and GetCacheEntryCommand to have good separation and avoid conditional logic in GetKeyValueCommand.

• As indicated by Pedro in https://github.com/infinispan/infinispan/pull/2671:

I had something simpler in mind. you could invoke create(cacheEntry) that returns a mutable copy. I think it is ok the copy to be mutable, but it you really want it immutable, you can do immutableInternalCacheEntry(create(cacheEntry))
this way you would avoid all the changes in CoreImmutables

• Finally, sort out TODO in documentation of InternalEntryFactory