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.

Shared db locking was used to avoid the case in which two different devices
try to store/modify remotelly stored secrets at the same time. We want to
avoid remote locks because of the problems they create, and prefer to crash
locally.

For the record, we are currently using the user's password to encrypt the
secrets stored in the server, and while we continue to do this we will have to
re-encrypt the secrets and update the remote storage whenever the user changes
her password.