Speedup monitoring - eliminate clone of histogram fillers

This MR eliminates the need to copy histogram fillers. As a tradeoff, the whole "setup" of fillers need to be locked. Initially, I got better timing (~50%) but it was in a version where the state in GMT caused it to fail true MT test. Once this got fixed (needed to delay retrieval of fillers to actually fill for the group) the filling is faster, but there is no breakthrough. I have another branch with even more drastic changes that make things another 20% faster. Tagging @fwinkl @stelzer @ponyisi @cburton

Relates to ATR-21210.

Timing before:

fillFromScalar                MON: 13097 +- 0 us        ROOT: 279 +- 0 us
fillFromMultipleScalars       MON: 27978 +- 0 us        ROOT: 1249 +- 0 us
fillFromCollection            MON: 12209 +- 0 us        ROOT: 1476 +- 0 us

and timing after:

fillFromScalar                MON: 8385 +- 0 us         ROOT: 218 +- 0 us
fillFromMultipleScalars       MON: 16063 +- 0 us        ROOT: 1163 +- 0 us
fillFromCollection            MON: 8659 +- 0 us         ROOT: 1259 +- 0 us

Both fluctuate a lot though. These are kind of typical.