ATEAM-909: Lock flavor tagging decorations

I'm trying to prevent the bug we had in AFT-646 from happening again. In that bug, we were overwriting a decoration we saved on the BTagging object in a subsequent algorithm. This was allowed because the algorithm itself called another tool that added decorations, but those decorations were never locked.

This MR fixes that. It was part of a pretty long saga where we:

• found that decoration locking wasn't working at all
• fixed it (well mostly Scott fixed it, and then fixed a few dozen places where decorations were being overwritten)
• forgot about this MR for a few years
• found several similar bugs, and finally
• revived this MR after Scott added some changes to the EDM to make the locking easier in the FTAG case.