Add read/write operation to store_test.cc

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Return 0 when the offset of read exceed the length of object

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Fix incorrect read and truncate

If the first chunk of object is not read entirely, the current implementation
will contain it entirely which make wrong.

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