add_metadata reimplemented as a locked transaction, so that multiple processes can update the same file with different metadata at the same time - makes bam_import_from_irods_and_vrtrack_qc.t pass consistently
add_metadata reimplemented as a locked transaction, so that multiple processes can update the same file with different metadata at the same time - makes bam_import_from_irods_and_vrtrack_qc.t pass consistently

wtsi-hgi/vr-pipe,wtsi-hgi/vr-pipe