Drop ReaderWriterLockSlim from MessageBus.

ReaderWriterLockSlim managed to get corrupted internal state and cause
deadlocks.

Apparently mono's implementation of it is very buggy:
https://bugzilla.xamarin.com/show_bug.cgi?id=13884
Drop ReaderWriterLockSlim from MemoryModelCache.

ReaderWriterLockSlim managed to get corrupted internal state and cause
deadlocks in MessageBus.

Apparently mono's implementation of it is very buggy:
https://bugzilla.xamarin.com/show_bug.cgi?id=13884
Drop ReaderWriterLockSlim from ModelManager.

ReaderWriterLockSlim managed to get corrupted internal state and cause
deadlocks in MessageBus.

Apparently mono's implementation of it is very buggy:
https://bugzilla.xamarin.com/show_bug.cgi?id=13884