Support New Versioning in WorkflowInfo.GetCurrentBuildID()

[ShahabT]

Is your feature request related to a problem? Please describe.

In new versioning, current build ID for tasks being replayed should be retrieved from the WFTStarted event rather than WFTCompleted event. The stamp in WorkflowTaskCompletedEventAttributes is deprecated and will be removed after old versioning clean up.

Note that if the started event does not have a (non-empty) Build ID, then the one in completed task should be still used to keep the support for old versioning for now.

[antlai-temporal]

Currently we are duplicating the Build ID and adding it to both WFTStarted and WFTCompleted for backwards compatibility, is that correct?