usage of multiple static variables is discouraged

[rhaschke]

I'm double-posting this issue, which shows up in pluginlib usage: https://github.com/ros/pluginlib/pull/37

Using static pluginlib ClassLoader defers lib unloading to program shutdown time. But as the order of static releases is undefined, the program might crash.

IMHO, class_loader shouldn't use a handful of unrelated static vars, but collect them within a singleton class, which is only freed when all loaded libs were unloaded.