streaming: Futurize estimate_partitions
streaming: Futurize estimate_partitions

The loop can take a long time if the number of sstables and/or ranges
are large. To fix, futurize the loop.

Fixes: #4005

Message-Id: <e83f38462e86fa863473086928aef1f8e13e4bbf@scylladb.com>

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