This is to just fetch the message, given it's message id.

This is for querying reasons, to see what is available.

This is to query broker for pending message ids (i.e. ones that
haven't yet started).

This is to send message id over the wire

- add 'testing' section to README
- add PGMQBrokerInitConnStr constructor to PGMQ broker

This is useful for tests

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