Load runtimes dynamically via go1.8 plugins

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

Add registration for more subsystems via plugins

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

Move content service to separate package

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

runtime.go

@@ -1,53 +1,6 @@
 package containerd
 
-import (
-	"fmt"
-	"sync"
-
-	"golang.org/x/net/context"
-)
-
-// NewRuntimeFunc is the runtime's constructor
-type NewRuntimeFunc func(root string) (Runtime, error)
-
-var runtimeRegistration = struct {
-	mu       sync.Mutex
-	runtimes map[string]NewRuntimeFunc
-}{
-	runtimes: make(map[string]NewRuntimeFunc),
-}
-
-// RegisterRuntime is not external packages registers Runtimes for use with containerd
-func RegisterRuntime(name string, f NewRuntimeFunc) {
-	runtimeRegistration.mu.Lock()
-	defer runtimeRegistration.mu.Unlock()
-	if _, ok := runtimeRegistration.runtimes[name]; ok {
-		panic(fmt.Errorf("runtime already registered as %q", name))
-	}
-	runtimeRegistration.runtimes[name] = f
-}
-
-// Runtimes returns a slice of all registered runtime names for containerd
-func Runtimes() (o []string) {
-	runtimeRegistration.mu.Lock()
-	defer runtimeRegistration.mu.Unlock()
-
-	for k := range runtimeRegistration.runtimes {
-		o = append(o, k)
-	}
-	return o
-}
-
-// NewRuntime calls the runtime's constructor with the provided root
-func NewRuntime(name, root string) (Runtime, error) {
-	runtimeRegistration.mu.Lock()
-	defer runtimeRegistration.mu.Unlock()
-	f, ok := runtimeRegistration.runtimes[name]
-	if !ok {
-		return nil, ErrRuntimeNotExist
-	}
-	return f(root)
-}
+import "golang.org/x/net/context"
 
 type IO struct {
 	Stdin    string
@@ -73,7 +26,7 @@ type Runtime interface {
 	// Containers returns all the current containers for the runtime
 	Containers() ([]Container, error)
 	// Delete removes the container in the runtime
-	Delete(ctx context.Context, c Container) error
+	Delete(context.Context, Container) error
 	// Events returns events for the runtime and all containers created by the runtime
 	Events(context.Context) <-chan *Event
 }