GMS: separate view bundling timeouts for JOIN, LEAVE, MERGE

View bundling in GMS bundles events based on (1) whether they can be processed together and (2) whether a view bundling timeout is defined. The latter timeout is for all events, perhaps we want to define separate timeouts, e.g. the timeout for JOINs is 3000, but for LEAVE it is 500. This means that when we get multiple JOIN events, we'll wait up to 5 seconds and then process them together. When we get multiple LEAVE requests, we only wait for half a second before processing them. This causes LEAVEs to be processed almost immediately, whereas JOINs are bundled.

Also revisit the code which determines which events can be processed together. Perhaps MERGE events can be bundled, too ?