Add random cache and replace SharedLRU in KeyValueStore

Add random cache and replace SharedLRU in KeyValueStore

SharedLRU plays pool performance in KeyValueStore with large header cache size,
so a performance optimized RandomCache could improve it.

RandomCache will record the lookup frequency of key. When evictint element,
it will randomly compare several elements's frequency and evict the least
one.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Conflicts:

	src/common/config_opts.h
	src/os/KeyValueStore.cc