Refactor finalization logic so that Exceptions are not thrown on finalizer thread

Reviewing MemcachedClient, there are a few issues that need to be resolved:

• The dispose/finalize logic itself is not correct in MemcachedClient. GCSuppressFinalize shouldn't be called during finalization for example.
• The finalizer contains code (logging) that could throw an exception if the config was not setup correctly or if It cannot create the instance for some reason