Fixing FindBugs warnings: MS_SHOULD_BE_FINAL

Basically adding "final" where it should be in first place (static non-private fields). Please do _not_ merge this PR yet, we need to discuss if all the changes are safe.

Most of the changes are trivial (e.g. adding final to logger object), but few of them could potentially break something, please see the comments inline
@sotty, could you please look at these setters? You added them some time ago (https://github.com/droolsjbpm/drools/commit/3858161e7c5f5ab6188667241beebb5c89ab8275), but they do not seem to be used. Is it safe to remove them?