vmemcache perf can't scaling with multi-thread.

[guoanwu]

Since the mutex, the multi-thread performance scaling is not good. With 4 threads, we see most of time consuming is on the mutex lock and unlock.
We run the benchmark:
[root@wilson03 benchmarks]# taskset -c 0-3 ./bench_micro /mnt/pmem0 put 4 10000000 104857600 4096 10 1024 1024