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>
I'm a -1 on the vague "properties" blob- instead I'd say we explicitly add fields we expect.

In that line, is there a 'server set version' proper tie somewhere? If we're starting to iterate on contents like this, we may benefit from explicit versioning to know what to expect ahead of time.