Make KeyValueStore support set_alloc_hint op

Add a new config let KeyValueStore support configurable strip size.
set_alloc_hint op can affect the strip size of the specified object
and the expect write size will become the strip size of the object.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add set_alloc_hint test to ceph_test_objectstore

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add upper limit to the write size of set_alloc_hint in KeyValueStore

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1937 from yuyuyu101/keyvaluestore-strip-size

Add strip size and set_alloc_hint op support for KeyValueStore

Reviewed-by: Sage Weil <sage@inktank.com>