Implement #1363 "Internal requests must not go through HTTP"

- new InternalHttpClient
  - uses LocalRequest and LocalResponse
  - same interface as HttpClient
- prologue-servlet.xpl
- Connection
  - uses InternalHttpClient when needed
  - uses StreamedContent
  - uses URI instead of URL
- refactor
  - don't pass headers as Array but as immutable List
  - move several classes to Scala
  - indent and named parameters
  - LocalResponse takes URLRewriter
  - Connection
  - ConnectionResult
- LocalRequest
  - sanitize
  - don't use RequestWrapper
- new uses of Connection
  - FormRunnerPersistenceProxy
  - XFormsResourceServer
- RequestDispatcherSubmission no longer used except in separate deployment
- EchoSubmission
  - echo headers
  - echo content type
- Defaults object
  - for more global default constants
  - remove duplication of some constants