[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-digest_amd64.go

@@ -0,0 +1,178 @@
+//+build !noasm,!appengine,gc
+
+// Copyright (c) 2020 MinIO Inc. All rights reserved.
+// Use of this source code is governed by a license that can be
+// found in the LICENSE file.
+
+package md5simd
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"sync/atomic"
+)
+
+// md5Digest - Type for computing MD5 using either AVX2 or AVX512
+type md5Digest struct {
+	uid         uint64
+	blocksCh    chan blockInput
+	cycleServer chan uint64
+	x           [BlockSize]byte
+	nx          int
+	len         uint64
+	buffers     <-chan []byte
+}
+
+// NewHash - initialize instance for Md5 implementation.
+func (s *md5Server) NewHash() Hasher {
+	uid := atomic.AddUint64(&s.uidCounter, 1)
+	blockCh := make(chan blockInput, buffersPerLane)
+	s.newInput <- newClient{
+		uid:   uid,
+		input: blockCh,
+	}
+	return &md5Digest{
+		uid:         uid,
+		buffers:     s.buffers,
+		blocksCh:    blockCh,
+		cycleServer: s.cycle,
+	}
+}
+
+// Size - Return size of checksum
+func (d *md5Digest) Size() int { return Size }
+
+// BlockSize - Return blocksize of checksum
+func (d md5Digest) BlockSize() int { return BlockSize }
+
+func (d *md5Digest) Reset() {
+	if d.blocksCh == nil {
+		panic("reset after close")
+	}
+	d.nx = 0
+	d.len = 0
+	d.sendBlock(blockInput{uid: d.uid, reset: true}, false)
+}
+
+// write to digest
+func (d *md5Digest) Write(p []byte) (nn int, err error) {
+	if d.blocksCh == nil {
+		return 0, errors.New("md5Digest closed")
+	}
+
+	// break input into chunks of maximum internalBlockSize size
+	for {
+		l := len(p)
+		if l > internalBlockSize {
+			l = internalBlockSize
+		}
+		nnn, err := d.write(p[:l])
+		if err != nil {
+			return nn, err
+		}
+		nn += nnn
+		p = p[l:]
+
+		if len(p) == 0 {
+			break
+		}
+
+	}
+	return
+}
+
+func (d *md5Digest) write(p []byte) (nn int, err error) {
+
+	nn = len(p)
+	d.len += uint64(nn)
+	if d.nx > 0 {
+		n := copy(d.x[d.nx:], p)
+		d.nx += n
+		if d.nx == BlockSize {
+			// Create a copy of the overflow buffer in order to send it async over the channel
+			// (since we will modify the overflow buffer down below with any access beyond multiples of 64)
+			tmp := <-d.buffers
+			tmp = tmp[:BlockSize]
+			copy(tmp, d.x[:])
+			d.sendBlock(blockInput{uid: d.uid, msg: tmp}, len(p)-n < BlockSize)
+			d.nx = 0
+		}
+		p = p[n:]
+	}
+	if len(p) >= BlockSize {
+		n := len(p) &^ (BlockSize - 1)
+		buf := <-d.buffers
+		buf = buf[:n]
+		copy(buf, p)
+		d.sendBlock(blockInput{uid: d.uid, msg: buf}, len(p)-n < BlockSize)
+		p = p[n:]
+	}
+	if len(p) > 0 {
+		d.nx = copy(d.x[:], p)
+	}
+	return
+}
+
+func (d *md5Digest) Close() {
+	if d.blocksCh != nil {
+		close(d.blocksCh)
+		d.blocksCh = nil
+	}
+}
+
+// Sum - Return MD5 sum in bytes
+func (d *md5Digest) Sum(in []byte) (result []byte) {
+	if d.blocksCh == nil {
+		panic("sum after close")
+	}
+
+	trail := <-d.buffers
+	trail = append(trail[:0], d.x[:d.nx]...)
+
+	length := d.len
+	// Padding.  Add a 1 bit and 0 bits until 56 bytes mod 64.
+	var tmp [64]byte
+	tmp[0] = 0x80
+	if length%64 < 56 {
+		trail = append(trail, tmp[0:56-length%64]...)
+	} else {
+		trail = append(trail, tmp[0:64+56-length%64]...)
+	}
+
+	// Length in bits.
+	length <<= 3
+	binary.LittleEndian.PutUint64(tmp[:], length) // append length in bits
+
+	trail = append(trail, tmp[0:8]...)
+	if len(trail)%BlockSize != 0 {
+		panic(fmt.Errorf("internal error: sum block was not aligned. len=%d, nx=%d", len(trail), d.nx))
+	}
+	sumCh := make(chan sumResult, 1)
+	d.sendBlock(blockInput{uid: d.uid, msg: trail, sumCh: sumCh}, true)
+
+	sum := <-sumCh
+
+	return append(in, sum.digest[:]...)
+}
+
+// sendBlock will send a block for processing.
+// If cycle is true we will block on cycle, otherwise we will only block
+// if the block channel is full.
+func (d *md5Digest) sendBlock(bi blockInput, cycle bool) {
+	if cycle {
+		select {
+		case d.blocksCh <- bi:
+			d.cycleServer <- d.uid
+		}
+		return
+	}
+	// Only block on cycle if we filled the buffer
+	select {
+	case d.blocksCh <- bi:
+		return
+	default:
+		d.cycleServer <- d.uid
+		d.blocksCh <- bi
+	}
+}