Task Status: Messages and Timestamps

[aluzzardi]

We should task statuses to include timestamps (see #320). It will be useful to know "this task is pending since ".

Another cool feature would be to make use of the state's message: "this task is pending since because: there are not enough resources in the cluster".

/cc @stevvooe @aaronlehmann

[stevvooe]

I propose a message field on TaskStatus.

[aluzzardi]

I concur - we currently have an err field on TaskStatus. We could just make that a message and use it not just for errors.

[stevvooe]

@aluzzardi Please add conditions for closing this issue.

[aluzzardi]

Example:

Wed Apr 20 14:27:44 PDT 2016 - NEW - created by a scale up of service web
Wed Apr 20 14:27:47 PDT 2016 - ALLOCATED
Wed Apr 20 14:27:49 PDT 2016 - PENDING - not enough resources available
Wed Apr 20 14:27:52 PDT 2016 - ASSIGNED - strategy: spread | node score: 375
Wed Apr 20 14:27:56 PDT 2016 - ACCEPTED
...
Wed Apr 21 15:17:13 PDT 2016 - FAILED - exit code 32