Combine standby() into occupyTask()

This PR is about combing standby() into occupyTask() in framework.

Naming change:
1. "failed" is renamed to "free"

Logical flow:
1. controller init will create numberOfTask free tasks.
2. each node will start by trying to get a task under /freeTask/ dir

A nitpick on #101 :
Ignoring the epoch mismatch logging on meta change notification.
Each epoch will have a new life cycle. This will generate a lot of "epoch mismatch" messages because many would get the old epoch.
Since the reason we add epoch synchronization to eliminate the traffic on sending data, we don't care much about meta change.
Also it make sense to change FailedDir to something else. AvailableDir?