Persist lazy vals on serialize, throw them away during deserialize about salat HOT 3 CLOSED

@Coalesce would have the following behaviour:

• a null number is 0
• a null String is ""
• if we can't figure out what it is, it becomes ""
• None is treated the same

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@persist implies one-way persistence:

• it persists when you serialize from Scala object to DBObject (or other form, such as Avro, Protobuf, JSON etc)
• it is ignored when you deserialize from DBObject (etc etc) to Scala object

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Support on both lazy and non-lazy vals.

@Coalesce must take a param "default" with type java.lang.Object and default value set to null. If not supplied, then supply a sensible non-null value. If supplied, then supply the given default value. This would allow us to support using @Coalesce on non-obvious types like dates or embedded objects.

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