Add aligned checkpointing mechanism to the Keyby framework.

[victorunique]

What I did for aligned checkpoint was based on the previous KeyBy framework:

• two sources + one operator + one sink
• click on one source will generate a checkpoint barrier (a picture of wood log);
• when the barrier arrives at the operator, its stream will pause and wait for the other barrier;
• click on the other source and trigger another barrier, when the second barrier arrived at the operator, all the blocked streams resume.