database::clear_snapshot(): added support for deleting all snapshots
database::clear_snapshot(): added support for deleting all snapshots

When 'nodetool clearsnapshot' is given no parameters it should
remove all existing snapshots.

Fixes issue #639

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>

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