Using single class-loader for internal agent classes and resources

[mcculls]

Notes:

• the separate class-loaders were hiding a potential startup issue involving JFR and the throwable instrumentation
• the previous assumption was it was always safe to try and create the ExceptionSampler singleton at any time - assuming that if it was called too early, the static final member would be temporarily null without affecting creation later on (ie. no side-effect such as the JVM remembering and throwing a class initializer error)
• we now only 'unlock' access to the ExceptionSampler once we know startup has reached a point where its dependencies are all ready
• some unused/unnecessary methods were removed
• the internal JAR protocol was removed and replaced by a build-time generated index to the appropriate sub-module
• some unnecessary indirection (leftover from before we added classes to the bootstrap class path) was removed