[feature] add worker task serialization logic (#2989)

* improved server shutdown with more precise shutdown of modules + deferring of ALL of it

* move delivery and workers into separate files

* add worker task model and Serialize() / Deserialize() methods for message types

* start adding message serialize / deserialize tests

* start adding test cases

* update body rewinding to rely on standard library mechanism of r.GetBody()

* remove request rewinding (http.Client{} should already handle this)

* standard library already handles rewinding

* improved code comment

* move the newPOST() function contents to prepare(), fits better with current API

* add Serialize() / Deserialize() implementations for Delivery{} type

* finish writing FromClientAPI sserialize / deserialize tests

* start adding FromFediAPI{} serialize / deserialize test cases

* fix FromFediAPI{} tests

* add tests for delivery message type

* fix repeat code

* missing license header

* use testrig status and accounts for marshal / unmarshaling tests

* add a specific test for checking account RSA keys are preserved

vendor/github.com/google/go-cmp/cmp/internal/flags/flags.go

@@ -0,0 +1,9 @@
+// Copyright 2019, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flags
+
+// Deterministic controls whether the output of Diff should be deterministic.
+// This is only used for testing.
+var Deterministic bool