Remove two unneeded properties

marker.internal_vertex and marker.all_children_leaves are no
longer need because of a change in logic that uses
marker.is_leaf instead.
Add Test to verify size of marker tree built

Add code to pass
Change consecutive ints to ordered rand strings for child id suffixes

Add a little extra key distribution to child markers, instead of
ACBC,0
ACBC,1
ACBC,2

it does something like :

ACBC,Dr0
ACBC,HT1
ACBC,a62
Change results order to maintain insertion order

Ensures results are in the order the tasks are inserted into a job
especially when an Async fans out, it's results will be pulled in order
before the result of the next Async.
Add document ion and refactor update_done

I could remove lines of update_done with a little
refactoring and also made code clearer.