Run checkstyle plugin during package scope

1. Enable checkstyle verification during package phase. Currently it validates and writes result to console and target/checkstyle-result.xml files. 
2. Verification does not fail the build. Once all issues are fixed we can set `<failOnViolation>false</failOnViolation>` to true
3. Checkstyle rules are based on Falcon's rules, but with some modifications. Main modifications are - javadoc not required for package scope, allow package scope variables, local final not needed everywhere etc.
4. Some checks disabled for test code