Improve build speed by building all jsii targets at the same time,
and using a special aggregate POM build for Java.

- Improves speed by not unnecessarily re-reading jsii assemblies
  for each individual build.
- Improves speed by using an aggregate POM to build all Java
  targets (can reuse JVM instances and compilation metadata by Maven).
- Improves wall-clock time by building all languages in parallel.

On my machine, reduces the time to pack the entire CDK repository
from 25min user/15min wall clock to 15min user/5 min wall clock.

…uilds

…uilds

…uilds