Update task interface to allow custom task names and display names.

Adding optional override properties, __jug_name__ and
__jug_display_name__ which may be used to explictly set the task name or
display name. __jug_name__ sets the name property, used to calculate
task hashes, while __jug_display_name__ sets the new display_name
property, which is only used in status displays.
Add newline to lock file.
Refactor task execution loop to support progressively passing barriers.

Task execution loop "wait cycle" logic caused worker starvation in cases
with barriers executed after long-running tasks. Processes reaching
barrier value while all previously declared tasks were executing would
pause in wait cycle while currently running tasks executed if any tasks
were in a waiting state. This causes starvation when evaluation of the
barrier would allow new tasks to be declared and executed.

Refactor execution loop to allow "execute" command function to setup
external wait cycle loop by overriding internal execution wait cycle
limit. This external wait cycle may re-evaluate the barrier, accessing
new tasks.

This commit removes logic associated with task selection in the inner
execution loop, this may be reintroduced if needed.