ArchiveManager should not use hardcoded thread name prefixes

The implementation of org.jivesoftware.openfire.archive.ArchiveManager uses an ExecutorService that spawns threads. The names of these threads are provided by a NamedThreadFactory that uses a hard-coded prefix.

If multiple instances of ArchiveManager are created, they'll all use that same prefix, which will lead to duplicate thread names. This will likely cause problems.

ArchiveManager should be adapted to allow the thread name prefix to be configurable, or at least be unique.