TrigMessageSvc: modernize and sync with Gaudi/master

The TrigMessageSvc is used for online running in the HLT. It is based on the Gaudi MessageSvc but with some features removed and others added, i.e. the ability to forward messages to the online messaging system (ERS). This MR is a complete re-base of the Run-2 TrigMessageSvc onto the current Gaudi master. The code is thread-safe and should provide most required features.

TODO in a future MR:

• Add monitoring histograms
• Separate message statistics for each run (requires gaudi/Gaudi!866 (merged))
• Message dispatch in separate thread a la InertMessageSvc (if needed)

Notes to reviewers:

• This is complete re-write so there isn't much point to look at the diff
• Large parts of the code are copied from Gaudi. And while the coding could be improved in some places, I preferred to keep the code close to the original where possible to simplify back-porting future changes if needed.
• There are some couts in this code. This is unavoidable as the messaging system can of course only be used once the MessageSvc is fully initialized.

cc ATR-19394