Allow BranchIDLists to be used and updated concurrently

[makortel]

Would be needed for the option 1 outlined in cms-sw/cmssw#44643 (comment) .

Solution could be copying the BranchdIDLists for the incoming data for which it needs to be updated, and using shared_ptr to destroy the earlier copy after the last user of it no longer needs it.

Currently cms-sw/cmssw#44643 seems to be converging on option 1, but even if the direction would change, this ability would be a step towards allowing concurrent files.