Use spotbugs for static code analysis and make the code base warning free

[cmnrd]

Now that we have formatting addressed, we should also introduce linting and enforce that the code is warning free. Gradle has a convenient plugin called checkstyle for linting java code. It supports various style guide rule sets, including Google's style guide. This should be a tremendous help in avoiding the most common errors such as the problems fixed in #1793. The biggest challenge, however, will be to fix all linter warnings in our existing code.

[lhstrh]

Would be cool to set this up for reactor-c as well...

[cmnrd]

For reactor-c you could just copy the clang-format and clang-tidy setup from reactor-cpp.