stacktraces should have associated timestamps for each entry

[KrishnaPG]

Currently there is only one timestamp in the options while the stacktraces is an array. For debugging the timing issues, it is preferable to have each of the stacktraces entry be associated with its own timestamp. For jobs that are repeated and failed often, this helps much.

[skeggse]

Oh, good idea! We'll need to be careful about how this intersects with other readers of the data model like arena, and figure out what a migration strategy would look like in practice.