v0.22.0: Options validation removal (#102)

Service already validates all the passed options. Validating them on the client leads to situations when workflows fail due to a bad option when they could be recovered. And even worse it leads to situations when validation logic on the client gets out of sync with the service.

Also fixed TestWorkflowEnvironment to support mock testing of activities by their string name without any registration. Also removed initialization of a session worker in TestWorkflowEnvironment unless requested.