Async log compaction

[bmizerany]

etcd stops the writing-world while compacting its log. This was done on purpose to allow us to focus on harder problems. We plan to fix this problem in 0.5. See also: #732.

[xiang90]

@barakmich @unihorn I will try to do this on our existing store first if it is reasonable. But more likely, we will move to a new store implementation soon.

[xiang90]

this has been solved by a4dab7a.

there are still some log compaction/snapshot related issue tracked by #2370