sstables: delete_atomically: don't drop futures
sstables: delete_atomically: don't drop futures

We still allow the delete of rows from system.large_partition to run
in parallel with the sstable deletion, but now we return a future that
waits for both.

Tests: unit (release)

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20190205001526.68774-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla