Process messages from the same chain concurrently

[cam-schultz]

Context and scope
When adding bootstrapping logic and state in #20, we identified issues with processing messages from the same source chain concurrently. The workaround was to process such messages in serial in order to provide stronger guarantees of block ordering within the application and database.

Discussion and alternatives
We should consider adding back in this concurrent processing of messages from the same source chain in order to potentially improve performance.

Open questions
Under what conditions will adding this concurrency improve performance? In any case, relayer message processing is capped by the rate at which messages are included in the source chain, so we should make sure this would yield performance improvements before pursuing, especially since the synchronization logic is likely to be complex.