auxiliary primary keys

[sullivan-]

#108382016 gives the user control over the database key that allows for fast partitioned access in a partitioned database. however, database users sometimes need to resort to constructing a second table if they need an auxiliary path to look up an aggregate quickly. (this secondary table essentially maps from the auxiliary key to the partition key of the main table.) add a PType.auxPrimaryKey construct that does this for the user, instead of them having to create an auxiliary "aggregate" to make this happen.

would want an auxPartitionIndex too..

[sullivan-]

on my story board here: https://www.pivotaltracker.com/story/show/107688542