Setting access scanner sleeptime caused memcached to crash

Setting alog_sleep_time sometimes would crash memcached process.

Thread 5 Crashed:
0 libsystem_c.dylib 0x00007fff8acd0f91 pthread_mutex_lock + 17
1 ep.0.so 0x000000010084f031 Mutex::acquire() + 19 (mutex.cc:67)
2 ep.0.so 0x000000010081ee7e EventuallyPersistentStore::setAccessScannerSleeptime(unsigned long) + 36 (ep.cc:2508)
3 ep.0.so 0x000000010089b193 Configuration::setParameter(std::string const&, unsigned long) + 551
4 ep.0.so 0x000000010089c95d Configuration::setAlogSleepTime(unsigned long const&) + 65
5 ep.0.so 0x000000010083fc2a processUnknownCommand + 4746 (ep_engine.cc:417)
6 ep.0.so 0x000000010083e4e2 EvpUnknownCommand + 50 (ep_engine.cc:978)
7 bucket_engine.0.so 0x000000010056cd38 bucket_unknown_command + 744 (bucket_engine.c:2428)
8 memcached 0x000000010044b4ae complete_nread_binary + 7630 (memcached.c:2878)
9 memcached 0x00000001004412bf conn_nread + 1247 (memcached.c:3819)
10 memcached 0x0000000100444055 event_handler + 645 (memcached.c:5930)
11 libevent-2.0.5.dylib 0x0000000100479eae event_base_loop + 1089
12 memcached 0x00000001004516ae worker_libevent + 62 (thread.c:301)
13 libsystem_c.dylib 0x00007fff8acd28bf _pthread_start + 335
14 libsystem_c.dylib 0x00007fff8acd5b75 thread_start + 13