Add findbugs reporting, including exclusions, and resolve some findbugs complaints

This is a proposal for consideration.  I'm perfectly fine if any or all of the changes are rejected as either unwise, unsafe, or undesirable.

I believe they are all safe, but am not expert enough in Jenkins development to be 100% confident that a field of a Serializable class can safely be declared transient with no negative side effects in remoting or in version upgrades.