Code Review and Action Items on ExtensionLoader/ServiceExtensionLoader

We should do a community review on the ExtensionLoader API and ServiceExtensionLoader implementation. This is the mechanism used to power Assignable.as();

• Archives API should not expose extension loading configuration (SHRINKWRAP-102)
• ServiceExtensionLoader needs documentation at class-level and on methods
• ServiceExtensionLoader is not Thread-safe (or documented that this is the responsibility of the client, ArchiveBase, which does not sync access)
• OK to always use TCCL at time of instantiation to find extension impls and classes?
• ServiceExtensionLoader instance needs to be carried in the state of every archive? Should some Assignable types be given access to different service extension loaders? To me this looks like a singular point of configuration possibly warranting use of a Singleton.
• All extension loading as a feature needs Wiki documentation