Make the PublisherService threadsafe

In PublisherService, methods like the following:

1. getAvailablePublicationTrees() and
2. getPublicationTree()

use HashMap and not ConcurrentHashMap. This means that concurrent updates to the map may corrupt them and lead to infinite loops and 100% CPU load.

Note: This should not affect 9.x branch since there was some refactoring done.