Refactor messaging.

[mwarzynski]

At the beginning I wanted to fix logging an error coming from processing a message (retry). And a couple of minutes later I was in the process of refactoring the whole package...

• I fixed the race condition related to setting readFunc in the Reader,
• I unified Reader constructors,
• I removed dependency to config provider (params might be provided by ReaderOptions),
• I added handling context in messages reader (we may stop reading messages when server stops properly)
• I added functionality of stopping reading from kafka reader (we may stop one reader and then start reading with another function),
• I added better logging (for example, all messages contain topic and have the same format)
• I added Brokers struct which allows to remove the duplications of code between our internal projects. We may just initialize this simple struct in the main function and use it to construct Readers/Writers without passing brokers as an argument.
• I removed blank lines from tests.

The changes are breaking backwards compatibility. Nevertheless, it should be easy to fix after the dependency upgrade.