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
@etirelli, is it safe to make the cache dir final? It seems, at least from the upper case usage, that this is a constant.