[FIXED JENKINS-23339] - Introduce the validated Jenkins::getActiveInstance() method

jenkins.model.Jenkins::getInstance() method is marked as @CheckForNull, but the most of Jenkins code does not actually check the return value. It leads to tons of static analysis errors, hence it is hard to analyze the code.

The change also adds annotations to old methods like Hudson::getInstance()

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>