Support for dates (parsing + serializing)

Meteor stores dates as timestamps in milliseconds. The appropriate Java type would be `long`.

But when a client communicates with the server, the server always sends them in another form and expects them in the same form when it receives data from the client.

What Meteor does is wrapping a plain timestamp such as `1459443366000` in a JSON object containing the date as a property, such as `{ "$date": 1459443366000 }`.

So when you receive a date, the value is not the timestamp itself but the JSON object that you must parse the date from, first.

And when you try to send a date, you can't just put the timestamp right in there but must wrap it in a JSON string, e.g. `myMap.put("myDate", "{ \"$date\": 1459443366000 }");`.

We should add built-in support to make this easier.

No files or tests are named in the issue. Start by locating the client’s receive and send serialization entry points, then compare their handling with Meteor’s {$date: milliseconds} representation. Done means dates can be parsed from that object and serialized back into the expected form without manual wrapping.

Written by the indexing model from the issue text.