timer_wordcap command

timer_wordcap is a toy streaming pipeline that demonstrates the use of State and Timers. Periodic Impulse is used as a streaming source that produces sequence of elements upto 5 minutes from the start of the pipeline every 5 seconds. These elements are keyed and fed to the Stateful DoFn where state and timers are set and cleared. Since this pipeline uses a Periodic Impulse, the pipeline is terminated automatically after it is done producing elements for 5 minutes.