ExecutableStageDoFnOperator should fire processing time timers properly instead of draining it directly when close() happens

[damccorm]

We hit this case when processing time timer holds the watermark but at the same time we are closing the operator.

One typical case is when SDF sets timer for rescheduling residuals.

Imported from Jira BEAM-11210. Original Jira may contain additional context.
Reported by: boyuanz.