Applications are not 1:1 with class loaders

[dmlloyd]

Description

As a:

• Application user/user of the configuration itself
• API user (application developer)
• SPI user (container or runtime developer)
• Specification implementer

...I need to be able to:

Break the 1:1 mapping between class loaders and configurations.

...which enables me to:

• Have an application which spans multiple class loaders, yet shares one configuration.
• Have a configuration which spans/is used by multiple applications.

Note that some containers rely on a 1:1 mapping; they should continue to be supported by any proposed solution.

Relates to #471, #472.