Modify async_server and async_connection to be SSL-only

- horrible hack to get some HTTPS support, better solution
  TBD (sub-classing or tag-based)

Problems:
- linking of unit tests referencing async_server fails with missing
  OpenSSL libs
- async_server gets ssl context optionally to get existing unit tests
  to compile, but async_connection constructor needs the context anyway
  -> tests of async_server will all fail