Theoretically (until now), Soledad inherits from U1DB the behaviour of only
accepting valid JSON for documents contents. JSON documents only allow for
unicode strings. Despite that, until now we had implemented lossy convertion
to unicode to avoid encoding errors when dumping/loading JSON content. This
allowed for API users to pass non-unicode to Soledad, but caused the
application to take more time because of conversion.

There were 2 problem with this: (1) conversion may take a long time and a lot
of memory when convertin large payloads; and (2) conversion was being made
before deferring to the adbapi, and this was blocking the reactor.

This commit completelly removes the conversion to unicode, thus leaving the
responsibility of unicode conversion to users of the Soledad API.