use unidoc plugin

The homemade solution was good, but a plugin exists, it is better to
reuse was is already there
add a method to get instance information

the client checks that its version is compatible with the real one and
warns if it is not the case
some refactoring

It is not necessary for the password hashing stuffs tu live all
CouchClient instances, thus it is better to put it in the package
directly.
The serializer just needs to know the version of couchdb to
(de)serialize for.