eventlogging.streams: drop mappers; pass URL parts.

* Refactor eventlogging.factory to make it pass URL parts to the target
  function as well.
* Refactor functions in eventlogging.handlers to utilize URL parts when doing
  so means the function has to do fewer manipulations on its input.
* Get rid of mappers, since that functionality is better implemented as
  optional keyword arguments to readers and writers.
* Removed unnecessary 'iter_socket' import in eventlogging-multiplexer.

renames:

* iter_socket -> iter_unicode
* iter_socket_json -> iter_json
* eventlogging.stream -> eventlogging.streams

Change-Id: I05f75db2b94c4d046a6ebc150e8f3b4c935d01bd