Improve isRunning() implementation for OrchestrationMetadata

[andystaples]

Currently, isRunning() relies on the name, instanceId, and runtimeStatus parsed from the OrchestrationState returned over gRPC to determine whether the orchestration is running, however, there is also an Exists field on OrchestrationState that should also be considered, as runtimeStatus is populated from an enum with a default value.

This may also affect the implementation of isCompleted() and/or isInstanceFound()