Add open flag to transfer ownership of array buffers

[jeromekelleher]

When kastore_gets returns an array buffer it's giving back a pointer that is owned by the kastore. After #175, all array pointers returned will be individually malloced buffers, which client code may wish to take ownership of to avoid extra copies.

We need to add a flag to kas_open() which indicates that a call to kastore_get involves the transfer of ownership of the buffer from kastore to the client. Once an item has been successfully returned to the client we set a flag on the struct saying "don't free this buffer", so that fields that are not retrieved don't result in a memory leak.

What do we call this flag? Say, KAS_GET_OWNS_ARRAY?

[benjeffery]

If we want to use similar language to tskit then I guess KAS_GET_TRANSFERS_OWNERSHIP?

[jeromekelleher]

Yes, good idea.