Crash in `hlt2_pp_thor` due to not thread safeness

While running throughput test hlt2_pp_thor a seg fault appeared, see https://lhcbpr-hlt.web.cern.ch/PerfTests/UpgradeThroughput/Throughput_lhcb-master-mr.8569_Moore_hlt2_pp_thor_x86_64_v3-centos7-gcc12+detdesc-opt+g_2023-07-17_05:55:40_+0200/tests.log

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related to !2338 (comment 6923871)

See Thread 3 and Thread 8

FYI: @graven