Doing upsert's individually.

Doing upserts as a batch was causing deadlocks in production when there were
overlapping tuples being upserted in different threads in different orders.

A script to reproduce this can be found here:
https://gist.github.com/drsnyder/6466651.

To prevent this, upsert each row individually. Of the many ways we may attempt
to solve this individually upserting seemed to me to be the simplest approach.

/cc @loganlinn @frankfarmer
Force realization of mset map before returning.

/cc @loganlinn
Merge pull request #22 from drsnyder/deadlock

Do upsert's individually to prevent deadlock