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>

services/server/config/config.go

@@ -55,6 +55,8 @@ type Config struct {
 	Cgroup CgroupConfig `toml:"cgroup"`
 	// ProxyPlugins configures plugins which are communicated to over GRPC
 	ProxyPlugins map[string]ProxyPlugin `toml:"proxy_plugins"`
+	// Timeouts specified as a duration
+	Timeouts map[string]string `toml:"timeouts"`
 
 	StreamProcessors []StreamProcessor `toml:"stream_processors"`