WIP Add support for custom torrents' approval storages

* migrate torrentapproval to list storage
* add initial support for torrent file storage (watch directory with fsnotify)
* replace frontend/http/bencode package with github.com/zeebo/bencode module
* sanitize code (fix warnings)

TODO:
* parse torrent files to get hashes,
* watch directory event types

DON'T use for now

middleware/torrentapproval/container/container.go

@@ -0,0 +1,53 @@
+package container
+
+import (
+	"errors"
+	"github.com/chihaya/chihaya/bittorrent"
+	"github.com/chihaya/chihaya/pkg/stop"
+	"gopkg.in/yaml.v2"
+	"sync"
+)
+
+type Builder interface {
+	New() (Container, error)
+}
+
+var (
+	buildersMU sync.Mutex
+	builders   = make(map[string]Builder)
+
+	ErrContainerDoesNotExist = errors.New("torrent hash container with that name does not exist")
+)
+
+func Register(n string, c Builder) {
+	if len(n) == 0 {
+		panic("middleware: could not register a Container with an empty name")
+	}
+	if c == nil {
+		panic("middleware: could not register a Container with nil builder")
+	}
+
+	buildersMU.Lock()
+	defer buildersMU.Unlock()
+	builders[n] = c
+}
+
+type Container interface {
+	stop.Stopper
+	Contains(bittorrent.InfoHash) bool
+}
+
+func GetContainer(name string, confBytes []byte) (Container, error) {
+	buildersMU.Lock()
+	defer buildersMU.Unlock()
+	var err error
+	var cn Container
+	if builder, exist := builders[name]; !exist {
+		err = ErrContainerDoesNotExist
+	} else {
+		if err = yaml.Unmarshal(confBytes, &cn); err == nil {
+			cn, err = builder.New()
+		}
+	}
+	return cn, err
+}