Add a reaction_idx field to the tracerecord

[erlingrj]

Currently, for tracepoints such as reaction_starts and reaction_ends, the reaction index is passed to the dst_id field of the tracerecord. This is a little inconsistent and the better approach would be to add a new field to the trace record called reaction_idx.

This would likely also need changes in the various trace_to_X utilities and fedsd and other external trace plugins.

[edwardalee]

One goal in this design is to fit as many trace points as possible into an in-memory buffer. I suggest instead changing the name of the field rather than increasing the footprint of each record.