more aggressive cancellation of broker parallel merge, more chill blo…

…cking queue timeouts, and query cancellation participation (apache#16748)

* more aggressive cancellation of broker parallel merge, more chill blocking queue timeouts

* wire parallel merge into query cancellation system

* oops

* style

* adjust metrics initialization

* fix timeout, fix cleanup to not block

* javadocs to clarify why cancellation future and gizmo are split

* cancelled -> canceled, simplify QueuePusher since it always takes a ResultBatch, non-static terminal marker to make stuff stop complaining about types, specialize tryOffer to be tryOfferTerminal so it wont be misused, add comments to clarify reason for non-blocking offers that might fail