SAMZA-859: Create a simple join example in hello-samza tutorial

[banecogic]

From Samza's State management documentation page:

Stream-stream join

Example: Join a stream of ad clicks to a stream of ad impressions (to link the information on when the ad was shown to the information on when it was clicked)

In this example we assume that each impression of an ad has a unique identifier, e.g. a UUID, and that the same identifier is included in both the impression and the click events. This identifier is used as the join key.

Implementation: Partition the ad click and ad impression streams by the impression ID or user ID (assuming that two events with the same impression ID always have the same user ID). The task keeps two stores, one containing click events and one containing impression events, using the impression ID as key for both stores. When the job receives a click event, it looks for the corresponding impression in the impression store, and vice versa. If a match is found, the joined pair is emitted and the entry is deleted. If no match is found, the event is written to the appropriate store. Periodically the job scans over both stores and deletes any old events that were not matched within the time window of the join.