214 question async consumerfunc

[dc1992]

• Replaced map() with a for to correctly manage async code in client
• ConsumerFunc returns void | Promise to handle async code
• Changed default credit policy to creditsOnChunkCompleted(1, 1)
• Updated tests to have an async function that handle messages
• Updated README.md

[tarzacodes]

The ConsumerFunc can be async or sync if I'm not mistaken - a lot of these changes, in the tests, are to substitute sync functions with async ones which makes the PR a lot harder to digest and check. Unless those changes in the tests are necessary to highlight some change in behaviour, I'd revert them