Optimize the trigger file discovery

The new implementation to discover omicron trigger files, GetOmicronFilePattern(), takes time. This implementation was introduced to manage 2 trigger areas, OFFLINE and ONLINE. This becomes non-negligible for applications dealing many channels (e.g. UPV). This ticket is an attempt to optimize the file discovery algorithm.

@nicolas.arnaud I suspect this is the reason why UPV/VetoPerf processes take so much time to complete. Do you have any indication to back-up my hypothesis? When running DQR, did you notice that the Omicron file discovery step takes an excessive amount of time?