Vine: Plot Exact Start Time of Task

[dthain]

When task groups are in effect, the manager sends multiple tasks to the worker at once, but they don't all start running right away. This results in misleading plots by vine_plot_txn_log which shows the task running right away.

The RUNNING state transition really means "was sent to the worker". The actual start time can be inferred from the wall_time property of the RETRIEVED message, working backwards. The tool should plot that instead.

Then once that's fixed, let's update the task vine-example logs repository appropriately.