The current code is assymetric: the first N-1 shards to delete a set receive
a synthetic future to wait on, while the last deletion receives the result
of the delete operation (which also broadcasts completion to the first N-1
operations. This results, in case of an error, with the Nth future being
reported as an unhandled error.
Fix by making everything symmetric: all N callers receive a synthetic
future. Nobody waits for the deletion operation (which still broadcasts its
completion to all waiters, so errors are not lost).
Message-Id: <20170305151607.14264-1-avi@scylladb.com>
