Initializes a new instance of the Update class

Modify value of local Replicator and replicate with given IWriteConsistency. The current value for the Key is passed to the Modify function. If there is no current data value for the Key the initial value will be passed to the Modify function. The optional request context is included in the reply messages. This is a convenient way to pass contextual information (e.g. original sender) without having to use `ask` or local correlation data structures.