* bump go-byteutil v1.2.0 -> v1.3.0 which has safer (as in long-term API consistency) byte <-> string conversions
* fix test relying on byteutil exported type no longer existing
* 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
