This commit renames the typeclass HasBroker into MessageBroker. The
`Has*` pattern is used in the context of mtl-style monadic stacks, where
we have "capabilities". Here we want to express a property of a concrete
type, so `MessageBroker` better encapsulate that.

This is hopefully better than dummy threadDelay's

All code copied from haskell-pgmq:
haskell-pgmq@8b05f5cc