properly expose AcquireByHash through the api

- new publish calls can now enable AcquireByHash by right away (previously
  one would have had to create a new publishing endpoint and then
  explicitly switch it to AcquireByHash)
- all json marshals of PublishedRepo now contain AcquireByHash (allows
  inspecting if a given endpoint has AcquireByHash enabled already; also
  enables verification that a switch/update actually applied a
  potential AcquireByHash change
- update all tests to reflect that default state of AcquireByHash
- update creation and switch testing to explicitly toggle AcquireByHash to
  make sure state mutation works as expected

deb/publish.go

@@ -315,6 +315,7 @@ func (p *PublishedRepo) MarshalJSON() ([]byte, error) {
 		"Sources":              sources,
 		"Storage":              p.Storage,
 		"SkipContents":         p.SkipContents,
+		"AcquireByHash":        p.AcquireByHash,
 	})
 }