Fix data race

[xiang90]

Fix data race.
The major fix is to clone the requests before sending them to different raft server.

[xiang90]

@benbjohnson This should fix #100.

[erikstmartin]

Awesome! Thanks Xiang, I was going to start peeking at it today 👍 :)

[xiang90]

@benbjohnson Maybe we can merge this?

[benbjohnson]

@xiangli-cmu Sorry I didn't get to this sooner. Two points of feedback:

1. I don't think we should expose setTerm() on an interface just for tests. Can you manually call s.(*server).mutex.Lock() in the tests instead and remove the setTerm() function? It's also strange because the initial-lowercase name will cause the function to not be exportable. I'm actually not sure how that would even work in Go.
2. I'm still seeing a data race for event#returnValue. Can you wrap this in a getter/setter function with a lock? Here's the race condition report: https://gist.github.com/benbjohnson/763b7e55fa8f59244ba5

[xiang90]

@benbjohnson For the second, I cannot understand it. I think it is already synced by the channel...
I will merge this.