colexec: fix recent regression with cancellation of inboxes

[yuzefovich]

Recent change 773d9ca fixed the way
inboxes handle regular query errors so that now the gRPC streams are not
broken whenever a query error is encountered. However, that change
introduced a regression - it is now possible that the inbox handler
goroutine (the one instantiated to handle FlowStream gRPC call) never
exits when the inbox is an input to the parallel unordered synchronizer.

In particular, the following sequence of events can happen:

1. the reader goroutine of the inbox receives an error from the
   corresponding outbox
2. this error is propagated to one of the input goroutines of the
   unordered synchronizer via a panic. Notably, this is considered
   a "graceful" termination from the perspective of the gRPC stream
   handling, so the handler goroutine is not notified of this error, and
   the inbox is not closed. It is expected that the inbox will be drained
   which will close the handler goroutine.
3. however, the synchronizer input goroutine currently simply receives
   the error, propagates it to the coordinator goroutine, and exits,
   without performing the draining.

Thus, we get into such a state that the inbox is never drained, so the
handler goroutine will stay alive forever. In particular, this will
block Flow.Wait calls, and Flow.Cleanup will never be called. This
could leak, for example, to leaking file descriptors used by the
temporary disk storage in the vectorized engine.

This fix is quite simple - instead of exiting in step 3, the
synchronizer's input goroutine should just proceed to draining, and only
exit once draining is performed. I believe this was always the
intention, but it didn't really matter until the fix in
773d9ca because draining of the inbox
was a noop since the gRPC stream was prematurely broken.

Fixes: #79469.

Release note: None

[cockroach-teamcity]

This change is 

[cucaroach]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @michae2)

[yuzefovich]

Fixed a typo in the comment.

TFTR!

bors r+

Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale) (waiting on @cucaroach and @michae2)

[craig]

Build succeeded:

• GitHub CI (Cockroach)