Delay loading metadata until it's really needed

Cause: XMvn spawns background threads to initialize some "eager"
singletons in order to minimize delay on their first use and improve
overall performance.

Consequence: Java allows isolated class realms to have multiple
singleton instances, so hundreds of initializer threads may be spawned
in applications heavily using isolated realms, such as in unit
testing.

Fix: Use lazy initialization instead of eager initialization.

Result: Lower performance, but better worst-case behavior.

Resolves: rhbz#1063301