sstables: make write_simple() safer by using exclusive flag
sstables: make write_simple() safer by using exclusive flag

We should guarantee that write_simple() will not try to overwrite
an existing file.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <194bd055f1f2dc1bb9766a67225ec38c88e7b005.1457818073.git.1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

avikivity/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla