Critical Description
Splitting out the patch on TEZ-1522 into a separate jira.
There's several scenarios in which we end up scheduling downstream tasks before their sources have been scheduled - and then get into a situation where the sources are starved. Currently, anywhere a ShuffleVertexManager is used can cause such behaviour - since it starts scheduling it's tasks after a certain number of sources are complete, but subsequen non-shuffle VertexManagers will scheduled immediately.
Disabling slow-start is one option to achieve this (or setting slow start on all vertices), but it doesn't work for the situation where dynamic reducer parallelism kicks in - since it has to wait for source tasks to complete.
The intent here is to add a DAGScheduler, which affectively negates the slow start, and in case of dynamic parallelism determination, waits for upstream tasks to be scheduled before scheduling downstream tasks.