SeekableStreamSupervisor: Don't await task futures in workerExec.

[gianm]

Following #17394, workerExec can get deadlocked with itself, because it waits for task futures and is also used as the connectExec for the task client. To fix this, we need to never await task futures in the workerExec.

There are two specific changes: in verifyAndMergeCheckpoints and checkpointTaskGroup, two coalesceAndAwait calls that formerly occurred in workerExec are replaced with Futures.transform (using a callback in workerExec instead).

Because this adjustment removes a source of blocking, it may also improve supervisor responsiveness for high task counts. This is not the primary goal, however. The primary goal is to fix the bug introduced by #17394.

[gianm]

Viewing this diff with whitespace hidden better illustrates what the changes are. Most of the lines changed are only indentation.