TrigOnlineMonitor: EventInfo const workarounds.

EventInfo has some const methods that return non-const pointers to members. We'd like to clean these up as part of the thread-safety work. However, this package relies on these in a couple places, to add stream tags to an existing EventInfo.

Presumably, this needs to be redone for run 3 anyway, so don't spend much effort on it at this point; instead, just put in some const_casts.