table: wait for pending streams on table::stop
table: wait for pending streams on table::stop

Stream sessions are now phased, so it's possible to wait for existing
streams to finish gently before stopping a table.

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