Allow timeouts to be configured in config

This adds a singleton `timeout` package that will allow services and user
to configure timeouts in the daemon.  When a service wants to use a
timeout, it should declare a const and register it's default value
inside an `init()` function for that package.  When the default config
is generated, we can use the `timeout` package to provide the available
timeout keys so that a user knows that they can configure.

These show up in the config as follows:

```toml
[timeouts]
  "io.containerd.timeout.shim.cleanup" = 5
  "io.containerd.timeout.shim.load" = 5
  "io.containerd.timeout.shim.shutdown" = 3
  "io.containerd.timeout.task.state" = 2

```

Timeouts in the config are specified in seconds.

Timeouts are very hard to get right and giving this power to the user to
configure things is a huge improvement.  Machines can be faster and
slower and depending on the CPU or load of the machine, a timeout may
need to be adjusted.

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

pkg/timeout/timeout.go

@@ -0,0 +1,66 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package timeout
+
+import (
+	"context"
+	"sync"
+	"time"
+)
+
+var (
+	mu       sync.Mutex
+	timeouts = make(map[string]time.Duration)
+
+	// DefaultTimeout of the timeout package
+	DefaultTimeout = 1 * time.Second
+)
+
+// Set the timeout for the key
+func Set(key string, t time.Duration) {
+	mu.Lock()
+	timeouts[key] = t
+	mu.Unlock()
+}
+
+// Get returns the timeout for the provided key
+func Get(key string) time.Duration {
+	mu.Lock()
+	t, ok := timeouts[key]
+	mu.Unlock()
+	if !ok {
+		t = DefaultTimeout
+	}
+	return t
+}
+
+// WithContext returns a context with the specified timeout for the provided key
+func WithContext(ctx context.Context, key string) (context.Context, func()) {
+	t := Get(key)
+	return context.WithTimeout(ctx, t)
+}
+
+// All returns all keys and their timeouts
+func All() map[string]time.Duration {
+	out := make(map[string]time.Duration)
+	mu.Lock()
+	defer mu.Unlock()
+	for k, v := range timeouts {
+		out[k] = v
+	}
+	return out
+}