shiny/driver/internal/event: do not over-retain references to events
shiny/driver/internal/event: do not over-retain references to events

The garbage collector considers slice elements past len to be
reachable, since the slice may be resliced to those elements at any
time. That means that elements past len(q.front) will remain reachable
until the next time len(q.front) again reaches its largest value.

Since we don't actually know when that will occur, nil out consumed
entries to allow their events to be collected as soon as possible.

Change-Id: I7bc914eebfd7dec42933b9fa5ad256d4db3701ab
Reviewed-on: https://go-review.googlesource.com/94135
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/exp,golang/exp,golang/exp