ATLAS in-file metadata and multi-threaded processing

<!--HTML-->Processing and scientific analysis of the data taken by the ATLAS experiment requires reliable information describing the event data recorded by the detector or generated in software. ATLAS event processing applications store such descriptive metadata information in the output data files along with the event information. To better leverage the available computing resources during LHC Run3 the ATLAS experiment has migrated its data processing and analysis software to a multi-threaded framework: AthenaMT. Therefore in-file metadata must support concurrent event processing, especially around input file boundaries. The in-file metadata handling software was originally designed for serial event processing. It grew into a rather complex system over the many years of ATLAS operation. To migrate this system to the multi-threaded environment it was necessary to adopt several pragmatic solutions, mainly because of the shortage of available person-power to work on this project in early phases of the AthenaMT development. In order to simplify the migration, first the redundant parts of the code were cleaned up wherever possible. Next the infrastructure was improved by removing reliance on constructs that are problematic during multi-threaded processing. Finally, the remaining software infrastructure was redesigned for thread safety.