athenaHLT: Fix creation of worker directories and MessageSvc format

Frank Winklmeierrequested to merge
fwinkl/athena:trigmessagesvc_mt into master

Two changes related to running in athenaHLT:

  • Add the slot and event number to the message printout
  • Delete the worker directory (and its content) in case it exists already. This is required to avoid a crash in the THistSvc.

cc @cyildiz

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