Reduce proposal submissions by add a proposals pool

[wuminzhe]

The extend-proposals does not need to be submitted immediately. We can add a extend-proposals pool for dj. dj submit these extend-proposals every few minutes.

So, if a new proposal comes in, dj can compare it with extend-proposals in the pool and check whether it can be ignored.

And, new coming extend-proposal also can be checked according to the pool.

[clearloop]

This part will be handled after the upgrade of the relay module.

Not necessary completely I think, but it's okay to let users customize their proposal-handling time themselves, for example:

• Default is submitting proposals immediately(or another param)
• Use dj -g <seconds> to set their proposal gaps