Creating Gaudi::Accumulators histograms/counters is not thread safe

On some occasions we need to create histograms after we see some events (i.e. in the operator()). We can lock this within each algorithm but different algorithms can still compete on creating histograms.

(In lhcb/LHCb!5078 (merged) this issue was solved by adding a global mutex, but that has problems on its own)