Improve LoKi::DistanceCalculator thread-safety

When running HLT2 multi-threaded, LoKi::DistanceCalculator raises an error:

ThreeBodyCombiner.LoKi::Distance...   ERROR LoKi::DistanceCalculator:: distance(III): error from KalmanFilter::step(2) StatusCode=405
HLTControlFlowMgr                      INFO Timing started at: 16:30:02
ThreeBodyCombiner.LoKi::Distance...   ERROR LoKi::DistanceCalculator:: The   ERROR message is suppressed : 'distance(III): error from KalmanFilter::step(2)' StatusCode=405

This is fixed by this MR. This MR should enable Moore to be run multi-threaded without writing output files, making a throughput test with hlt2 selections possible (lhcb-datapkg/PRConfig!184 (merged)). Further work will be necessary to actually make LoKi::DistanceCalculator thread-safe.

FYI: @sstahl @rmatev

Needs LHCb!3118 (merged) and DaVinci!570 (merged) (ref update).