Add test for omap interface

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add get_*_with_header to StripObjectHeader

In some situations, we need to ensure header is held and try to read, so read
interfaces need to accept header argument to allow.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add header cache to GenericObjectMap

Header plays a critical role in KeyValueStore, in order to improve performance
to reduce much calls to backend store cache IO path or touch actual IO, cache it
at GenericObjectMap. It will bring on much performance improvement for
KeyValueStore.

Add a new option "keyvaluestore_header_cache_size" to control the size of cache.

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