Overview issue: Things to improve in consul-haskell

[nh2]

Here I'm making a short list of key things to improve in consul-haskell.

• Document all functions.
• Provide some examples for common distributed-systems tasks, such as running a service with a health check from Haskell, or running a master-elected task across multiple Haskell servers.
• Sensibly group funciton docs into sections of tasks that you usually want to accomplish with consul.
• Use of Bool on errors.
  For example registerService :: ... -> m Bool returns False on any non-HTTP-200 response, discarding any useful error message, thus making it unclear how the caller should handle that case. Invalid request, or temporary loss of cluster leadership? The caller cannot tell.
  Some functions don't seem to check the status code at all; we need to fix that.
• Clean up comment this is here instead of the where to prevent typechecking nastiness
• Move the datacenter into ConsulClient (like the Python bindings do).
  Currently most functions take Maybe Datacenter as an argument, but it is usually not used, and if used, usually the same.
• Fix usage of plain async.
  That function isn't exception-safe (see first section of the main docs of the async package); use withAsync or other safe helper utilities that the docs instruct to use instead.
• Extract common code for functions that make requests.
  Most functions have very similar implementation:

  someFunctionality :: MonadIO m => ConsulClient -> FunctionalitySpecificRequestType -> m (Maybe FunctionalitySpecificResponseType)
  someFunctionality client@ConsulClient{..} request dc = do
    let hostnameWithScheme = hostWithScheme client
    initReq <- createRequest hostnameWithScheme
                             ccPort
                             "/v1/functionalitySpecificPath"
                             Nothing
                             (Just $ BL.toStrict $ encode request)
                             False
                             dc
    liftIO $ withResponse initReq ccManager $ \ response -> do
      case responseStatus response of
        x | x == status200 -> do
          bodyParts <- brConsume $ responseBody response
          return $ decode $ BL.fromStrict $ B.concat bodyParts
        _ -> return Nothing

  We should extract this common pattern into a helper function to make them easier to read.
• Implement ACL support.
• Add Github Actions based CI (Travis will discontinue OSS CI)
• Add tests for all functions, ideally also including coverage reports.

[ketzacoatl]

I've added a PR to address the Datacenter improvement.