Clean up distribution packaging

The distribution build process is currently very messy:

• it is based on overly complex assembly files
• the layout of the distribution packages is not ideal
• some of the documentation is not obviously accessible (jmx, xsd)
• since the javadoc plugin is configured to use aggregate builds, it cannot be run without building the entire tree

The solution:

• move the distribution logic to its own module
• use ant instead of assembly descriptors to build the distribution packages
• improve the layout, include the missing docs and remove the aging cruft