This review commit removes a bunch of redundant typeclass constraints,
especially the ones on the data declarations, generally considered a
codesmell.

Consider using newtypes for `Queue` and `TimeoutS`, for obvious reasons.
Also, use `Text` as an internal representation for `Queue`, as `String`
is just a list in disguise and thus quite slow in its cons-snoc semantic
(unlike `Text`, which efficiently represent textual data). `Text` also
supports UTF-16, which might be preferrable in some situations.

The use of DerivingStrategies makes it possible to print and render
queues in a "smart" way, for example:

  putStrLn $ show $ T.unpack $ _Queue (Queue "foo")
  putStrLn $ show (Queue "foo")

They both output "foo", without the intermediate `Queue` constructor one
would get by normal (stock) derivation.

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