Add SLF4J

[acogoluegnes]

Fixes #114

[acogoluegnes]

We now have a bunch a stack traces bubbling up during the test suite, because some errors were not logged at all in the exception handler. They should be harmless, as they occur during tests of error scenario. Let's see after a few builds if we should remove them, to make the CI reports easier to read.

[michaelklishin]

@acogoluegnes this can be handled separately but I think java.net.SocketExceptions with reason "Socket closed" and even "Connection reset" should be logged as warnings: they are just a fact of life in any production system. Logging them as errors will scare too many users for no good reason.

So logging them as warnings and maybe without full stack trace would be sufficient. Feel free to look into it in a follow-up PR.