Update reaper for multipe subscribers

Depends on https://github.com/containerd/go-runc/pull/24

The is currently a race with the reaper where you could miss some exit
events from processes.

The problem before and why the reaper was so complex was because
processes could fork, getting a pid, and then fail on an execve before
we would have time to register the process with the reaper.  This could
cause pids to fill up in a map as a way to reduce the race.

This changes makes the reaper handle multiple subscribers so that the
caller can handle locking, for when they want to wait for a specific
pid, without affecting other callers using the reaper code.

Exit events are broadcast to multiple subscribers, in the case, the runc
commands and container pids that we get from a pid-file.  Locking while
the entire container stats no longs affects runc commands where you want
to call `runc create` and wait until that has been completed.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux/shim/init_state.go

@@ -6,6 +6,7 @@ import (
 	"context"
 
 	"github.com/containerd/console"
+	"github.com/containerd/containerd/errdefs"
 	shimapi "github.com/containerd/containerd/linux/shim/v1"
 	"github.com/pkg/errors"
 )
@@ -345,10 +346,7 @@ func (s *stoppedState) Delete(ctx context.Context) error {
 }
 
 func (s *stoppedState) Kill(ctx context.Context, sig uint32, all bool) error {
-	s.p.mu.Lock()
-	defer s.p.mu.Unlock()
-
-	return s.p.kill(ctx, sig, all)
+	return errdefs.ToGRPCf(errdefs.ErrNotFound, "process %s not found", s.p.id)
 }
 
 func (s *stoppedState) SetExited(status int) {