Random TC Maker: logic of time for TC creation

[MRiganSUSX]

Current logic of Random TC candidate maker:

• get current timestamp estimate & pass it onto create_candidate function:
  https://github.com/DUNE-DAQ/trigger/blob/develop/plugins/RandomTCMakerModule.cpp#L265-L270
• create candidate then created new TC with readout window 'around' this timestamp:
  https://github.com/DUNE-DAQ/trigger/blob/develop/plugins/RandomTCMakerModule.cpp#L205-L219

This results in readout window requests that are in the future.

A better solution would be to shift the the timestamp slightly towards the past, ideally by adding a configurable offset.

[eflumerf]

Related: DUNE-DAQ/appmodel#244
Note that for the SNB demonstrator, I have a "PreconfiguredTriggerModule" which creates based on a given start and end time. There, the time_before and time_after offsets are applied not to a single point in time, but an already-existing window.
Moving to a "window_width" and "window_offset" model would work for this request, but would not work for the Preconfigured triggers; allowing the time_before and time_after to go negative would allow for offsetting the windows into the past (via negative time_after), while retaining flexibility for other TC maker types that work with windows instead of single points in time.

[wesketchum]

Marking done as all PRs merged.

[wesketchum]

Re-opened this we had to revert the commits due to failures in testing.