Support MongoDB map/reduce capabilities through JDO/JPA query interface

[andyjefferson]

The MongoDB java driver has mapReduce() methods taking in the reduction definition. We need to take in the reduction definition, perhaps as query extension/hint(s).

User has own conversion function for data perhaps, so used as bulk update type thing.

On a general note, with GAE v1 it stored data in a particular mode (with FK type info in elements). In v2 it is now stored in a new mode (with child information in property in the parent. To move from v1 to v2 a migration process would be a particular map function. Again, like a bulk update.

Obviously people more familiar (than me) with map-reduce will likely have their own uses; things that can't be expressed using JDOQL/JPQL maybe