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

This change:
* Introduces new @Nonnull jenkins.model.Jenkins::getActiveInstance() method
* Converts all unchecked calls within the core to the new method call

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