feat(store/store_bench_test.go) add a benchmark for set operation of store pkg
We randomly generage N 3 level keys. We benchmark the speed of setting each key into etcd store.
add a setWithJson test
add mem stats for sets benchmark
feat(store_bench) add set bench for different value sizes
add delete bench
Merge pull request #480 from xiangli-cmu/store_bench

feat(store/store_bench_test.go) add a benchmark for set operation