Direct runner could display the watermark for debugging

When using the direct runner to test and investigate streaming pipeline, it would be very convenient to display the watermark of each element to check if a window/trigger definition is correct.

Imported from Jira [BEAM-3278](https://issues.apache.org/jira/browse/BEAM-3278). Original Jira may contain additional context.
Reported by: jbonofre.

Start with the direct runner and review the imported BEAM-3278 Jira issue for the additional context. Trace how a streaming pipeline processes watermarks, windows, and triggers; done means the direct runner displays each element's watermark so those definitions can be inspected.

Written by the indexing model from the issue text.