Improvements to code involving synchronized maps

[ghost]

These are some improvements to org.apache.jackrabbit.jcr2spi.ItemImpl and other classes which use synchronized maps.

The listeners member of ItemImpl should be made private because subclasses should not be allowed to mutate this map or iterate over it. Iterating over the map requires proper synchronization or else non-deterministic behavior can occur.

Also, the changes to throw NullPointerException if an ItemLifeCycleListener instance is null is not really a change, because it was implicitly assumed that the listeners would not be null (notifyCreated() invokes itemCreated() on each initial listener without checking for null).

I would classify these changes as efficiency and code improvements, with three exceptions that fix concurrency issues:

• "Need to synchronize on docNumber2id for both get() and put()"
• "Fix potential non-determinism in DbDataStore.deleteAllOlderThan()"
• "Apply improvements from ItemImpl to CacheManager"