Split `log4j-tomcat` in smaller artifacts

[ppkarwasz]

Log4j Core plugins and Tomcat components need to be in different classloaders:

• Log4j core plugins should be in the system classloader,
• Tomcat components must be in the server/common classloader.

In order to be usable both in a webapp and as global logging system, we might split it into more artifacts:

• log4j-tomcat will contain only components that depend on Tomcat libraries and must be in the server/common classloader or its descendants,
• log4j-plugins-tomcat will contain Log4j Core plugins that depend on log4j-core.