fix active child tasks panic with endpoint.ListAbstractionsStreamed

The goroutine that does endTask() for
"list-abstractions-streamed-producer" can be preempted
after it has closed the out and outErrs channel,
but before it calls endTask().
If the parent ("handler") then gets scheduled and
and ends itself, it will observe an active child task
"list-abstractions-streamed-producer".

This is easy to demo by injecting a sleep here:

  --- a/endpoint/endpoint_zfs_abstraction.go
  +++ b/endpoint/endpoint_zfs_abstraction.go
  @@ -575,6 +576,7 @@ func ListAbstractionsStreamed(ctx context.Context, query ListZFSHoldsAndBookmark
          ctx, endTask := trace.WithTask(ctx, "list-abstractions-streamed-producer")
          go func() {
                  defer endTask()
  +               defer time.Sleep(10 * time.Second)
                  defer close(out)
                  defer close(outErrs)

fixes https://github.com/zrepl/zrepl/issues/607

client/zfsabstractions_list.go

@@ -44,10 +44,11 @@ func doZabsList(ctx context.Context, sc *cli.Subcommand, args []string) error {
 		return errors.Wrap(err, "invalid filter specification on command line")
 	}
 
-	abstractions, errors, err := endpoint.ListAbstractionsStreamed(ctx, q)
+	abstractions, errors, drainDone, err := endpoint.ListAbstractionsStreamed(ctx, q)
 	if err != nil {
 		return err // context clear by invocation of command
 	}
+	defer drainDone()
 
 	var line chainlock.L
 	var wg sync.WaitGroup