split workflow & task logs, add state and update time to task logs

[essweine]

This PR splits the spitt logger into task and worklfow logs and standardized attributes on both.

Attributes available for tasks:

• workflow_spec: the name of the workflow spec
• task_spec: the name of the task spec
• task_id: the task id
• task_type: a string representation of the tasks's class
• state: the name of the task's state
• last_state_change: the update timestamp

If the logging level is < 20 (below INFO)

• data: the task data
• internal_data: the task's internal data

Attributes available for workflows:

• workflow_spec: the spec name
• success: the value of the success attribute
• completed: the value of the completed attribute

If the logging level is < 20:

• tasks: a list of the task ids

The spiff.metrics logger has been removed.