[feature] S3 support (#674)

* feat: vendor minio client

* feat: introduce storage package with s3 support

* feat: serve s3 files directly

this saves a lot of bandwith as the files are fetched from the object
store directly

* fix: use explicit local storage in tests

* feat: integrate s3 storage with the main server

* fix: add s3 config to cli tests

* docs: explicitly set values in example config

also adds license header to the storage package

* fix: use better http status code on s3 redirect

HTTP 302 Found is the best fit, as it signifies that the resource
requested was found but not under its presumed URL

307/TemporaryRedirect would mean that this resource is usually located
here, not in this case

303/SeeOther indicates that the redirection does not link to the
requested resource but to another page

* refactor: use context in storage driver interface

vendor/github.com/minio/md5-simd/md5.go

@@ -0,0 +1,57 @@
+package md5simd
+
+import (
+	"crypto/md5"
+	"hash"
+	"sync"
+)
+
+const (
+	// The blocksize of MD5 in bytes.
+	BlockSize = 64
+
+	// The size of an MD5 checksum in bytes.
+	Size = 16
+
+	// internalBlockSize is the internal block size.
+	internalBlockSize = 32 << 10
+)
+
+type Server interface {
+	NewHash() Hasher
+	Close()
+}
+
+type Hasher interface {
+	hash.Hash
+	Close()
+}
+
+// md5Wrapper is a wrapper around the builtin hasher.
+type md5Wrapper struct {
+	hash.Hash
+}
+
+var md5Pool = sync.Pool{New: func() interface{} {
+	return md5.New()
+}}
+
+// fallbackServer - Fallback when no assembly is available.
+type fallbackServer struct {
+}
+
+// NewHash -- return regular Golang md5 hashing from crypto
+func (s *fallbackServer) NewHash() Hasher {
+	return &md5Wrapper{Hash: md5Pool.New().(hash.Hash)}
+}
+
+func (s *fallbackServer) Close() {
+}
+
+func (m *md5Wrapper) Close() {
+	if m.Hash != nil {
+		m.Reset()
+		md5Pool.Put(m.Hash)
+		m.Hash = nil
+	}
+}