KeyValueStore: Reduce redundancy set_header call

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
doc: Add keyvaluestore config description

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
KeyValueStore: Reduce redundant "make_pair"

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #2502 from yuyuyu101/kvstore-meta-cache

KeyValueStore: performance improvement and docs