Add Header cache to KeyValueStore

Add Header cache to KeyValueStore

In the performance statistic recently, the header lookup becomes the main time
consuming for the read/write operations. Most of time it occur 50% to deal with
header lookup, decode/encode logics.

Now adding header cache using SharedLRU structure which will maintain the header
cache and caller will get the pointer to the real header. It also avoid too much
header copy operations overhead.

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

	src/os/KeyValueStore.cc
	src/os/KeyValueStore.h