[Bug] uuid not defined in Persist::PostresPlugin#object_doc_get_by_uuid.
Ensure Ruby symbols are (de)serialized in PostgresPlugin.

In the MongoPlugin Ruby symbols are preserved in hash values thanks to
BSON, but in other persistence plugins symbols will be coerced to
strings (via a JSON.generate in the Postgres case) before saving. When
the object is deserialized the values that were Ruby symbols are now
strings which is not expected by the rest of the codebase.

Instead, we can "encode" the hash data structures containing symbols by
transforming any symbols to strings, for example: :symbol -> ":symbol".
When the data is read back out of the database we can "decode" these
data structures and reconstitute the symbols.