Send DB backend messages by partition instead of partition_key

[dorner]

Currently, DB backend producers can scale out the number of topics but not a single large topic with lots of partitions. To ensure order, only a single thread can send messages from a topic right now.

We should be changing this so that we save the partition of a message instead of a partition_key (using RubyKafka's Partitioner class to generate the partition). We should then add a configuration to allow locking particular topics (or all topics) by partition instead of just by topic. (For small topics this would be overkill.)

This will require a PR to Phobos to allow sending messages with a partition, which it currently does not support, as well as changing the save logic, the read logic, and the migration template.