Fix warning CA2002: Do not lock on objects with weak identity

[zerhacken]

This commit fixes warning CA2002. HierarchicalKeyStore extends
MarshalByRefObject which allows code code running in one AppDomain
to lock on an object in another AppDomain. Fix is to lock on a private
object which only HierarchicalKeyStore is knowledgeable about.

[gabikliot]

Hi

Thank you for this fix. 2 comments:

1. We usually use the "lockable" to name the private object to lock on, instead of mutex. Just our internal naming convention.
2. I think the original code in GrainTypeManager was correct. The lock has to be taken on a static field, since it has to be the same lock across different instances of the GrainTypeManager class. This is part of the singleton pattern.

[zerhacken]

1. Will rename "mutex" to "lockable"
2. You are right, I will fix this to lock on a static field.

I will squash the commits on dev/fix-storage-weak-identity-lock and provide a decent commit message.

[zerhacken]

Looks like the build failed "java.io.IOException: An existing connection was forcibly closed by the remote host"