Allow properties dict along within a ServiceInstance

When joining a ServerSet, it can be handy to attach some extra data to
the published ServiceInstance (i.e.: debugging info, service version,
etc.). This is expressed in the form of a <str, str> properties dict.

This only takes effect when reading/writing JSON (for now).

Signed-off-by: Raul Gutierrez S <rgs@twitter.com>