Load agent from dedicated agent class loader

[felixbarny]

Stop adding all the agent classes into the bootstrap class loader

• Hides the agent from the regular classpath
• Reduces risk of interfering with classpath scanners

Depends on #1337
Prerequisite for #1339

[eyalkoren]

When implementing this, consider using the agent class loader for a dedicated log4j context - in LoggingConfiguration#init we may want to switch to using org.apache.logging.log4j.core.config.Configurator#initialize(ClassLoader, Configuration) instead of the current org.apache.logging.log4j.core.config.Configurator#initialize(Configuration) and use that to further separate agent loggers from others.