Closes #603

This adds logging facilities at the shim level to provide minimal I/O
overhead and pluggable logging options.  Log handling is done within the
shim so that all I/O, cpu, and memory can be charged to the container.

A sample logging driver setting up logging for a container the systemd
journal looks like this:

```go
package main

import (
	"bufio"
	"context"
	"fmt"
	"io"
	"sync"

	"github.com/containerd/containerd/runtime/v2/logging"
	"github.com/coreos/go-systemd/journal"
)

func main() {
	logging.Run(log)
}

func log(ctx context.Context, config *logging.Config, ready func() error) error {
	// construct any log metadata for the container
	vars := map[string]string{
		"SYSLOG_IDENTIFIER": fmt.Sprintf("%s:%s", config.Namespace, config.ID),
	}
	var wg sync.WaitGroup
	wg.Add(2)
	// forward both stdout and stderr to the journal
	go copy(&wg, config.Stdout, journal.PriInfo, vars)
	go copy(&wg, config.Stderr, journal.PriErr, vars)

	// signal that we are ready and setup for the container to be started
	if err := ready(); err != nil {
		return err
	}
	wg.Wait()
	return nil
}

func copy(wg *sync.WaitGroup, r io.Reader, pri journal.Priority, vars map[string]string) {
	defer wg.Done()
	s := bufio.NewScanner(r)
	for s.Scan() {
		if s.Err() != nil {
			return
		}
		journal.Send(s.Text(), pri, vars)
	}
}
```

A `logging` package has been created to assist log developers create
logging plugins for containerd.

This uses a URI based approach for logging drivers that can be expanded
in the future.

Supported URI scheme's are:

* binary
* fifo
* file

You can pass the log url via ctr on the command line:

```bash
> ctr run --rm --runtime io.containerd.runc.v2 --log-uri binary://shim-journald docker.io/library/redis:alpine redis
```

```bash
> journalctl -f -t default:redis

-- Logs begin at Tue 2018-12-11 16:29:51 EST. --
Mar 08 16:08:22 deathstar default:redis[120760]: 1:C 08 Mar 2019 21:08:22.703 # Warning: no config file specified, using the default config. In order to specify a config file use redis-server /path/to/redis.conf
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.704 # You requested maxclients of 10000 requiring at least 10032 max file descriptors.
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.704 # Server can't set maximum open files to 10032 because of OS error: Operation not permitted.
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.704 # Current maximum open files is 1024. maxclients has been reduced to 992 to compensate for low ulimit. If you need higher maxclients increase 'ulimit -n'.
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.705 * Running mode=standalone, port=6379.
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.705 # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128.
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.705 # Server initialized
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.705 # WARNING overcommit_memory is set to 0! Background save may fail under low memory condition. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.705 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latency and memory usage issues with Redis. To fix this issue run the command 'echo never > /sys/kernel/mm/transparent_hugepage/enabled' as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled.
Mar 08 16:08:22 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:22.705 * Ready to accept connections
Mar 08 16:08:50 deathstar default:redis[120760]: 1:signal-handler (1552079330) Received SIGINT scheduling shutdown...
Mar 08 16:08:50 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:50.405 # User requested shutdown...
Mar 08 16:08:50 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:50.406 * Saving the final RDB snapshot before exiting.
Mar 08 16:08:50 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:50.452 * DB saved on disk
Mar 08 16:08:50 deathstar default:redis[120760]: 1:M 08 Mar 2019 21:08:50.453 # Redis is now ready to exit, bye bye...
```

The following client side Opts are added:

```go
// LogURI provides the raw logging URI
func LogURI(uri *url.URL) Creator { }
// BinaryIO forwards contianer STDOUT|STDERR directly to a logging binary
func BinaryIO(binary string, args map[string]string) Creator {}
```

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