Improve multi-threading for the handling of the messages

Could be improved with the following:

• Use the pooling mechanisme of ActiveMQ (BITMAG-357)
• Make setting for configuring the pooling of the threads.
• Make it possible to have separate pools e.g. for the ReferencePillar:
  • One pool for fast operations (e.g. identifies, GetStatus, etc.)
  • One pool for slow operations (e.g. file transfer, GetChecksums, Processing, etc.)