Transient fields

[JonathanMace]

Bags should support transient fields. Transient fields are omitted during serialization; that is, they only live within a single process.

Transient fields should behave like other fields -- eg they should be bags, and support merge/join

Since transient fields are not serialized, transient fields do not need a field number. They could be declared like:

int64 myregularid = 0;
int64 mytransientid = transient;

That's a little ugly but i think it'll do for now