Remove the near-useless test based on the Proton C client command line tool and replace it with the beginning of a test based on the Proton Java client.
Switch to representing symbols inside the plugin as binaries not printable-lists.
Small bugfixes to the messaging layer:

 * Correctly map timestamp <-> creation_time and expiration <-> ttl, as we document we do!
 * Never try to guess types in properties, the specs tell us what the types must be.
 * Therefore don't break if the symbol type is used (content_type or content_encoding).
Merge bug26288
stable to default