fastcgi: Optimize FastCGI transport (#4978)

* break up code and use lazy reading and pool bufio.Writer

* close underlying connection when operation failed

* allocate bufWriter and streamWriter only once

* refactor record writing

* rebase from master

* handle err

* Fix type assertion

Also reduce some duplication

* Refactor client and clientCloser for logging

Should reduce allocations

* Minor cosmetic adjustments; apply Apache license

* Appease the linter

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

modules/caddyhttp/reverseproxy/fastcgi/header.go

@@ -0,0 +1,32 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fastcgi
+
+type header struct {
+	Version       uint8
+	Type          uint8
+	ID            uint16
+	ContentLength uint16
+	PaddingLength uint8
+	Reserved      uint8
+}
+
+func (h *header) init(recType uint8, reqID uint16, contentLength int) {
+	h.Version = 1
+	h.Type = recType
+	h.ID = reqID
+	h.ContentLength = uint16(contentLength)
+	h.PaddingLength = uint8(-contentLength & 7)
+}