In order to check if a snapshot of a certain name exists the checking
method opens directory. It can be made with more lightweight call.
Also, though not critical, is that it fogets to close it.
Coroutinuze the method while at it.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closesscylladb/scylladb#18365