Fix message decoding to support custom tagged literals [IMMUTANT-350]

Clojure versions 1.4.0 or greater should autoload data_readers.clj and
both the :clojure and :edn codecs should work with any custom tags
defined there. For 1.3.0, clojure.tools.reader can be used.

Credit Jon Smock for the app used in the integration tests.