test: sstables test_env: introduce  do_with_async_returning()
test: sstables test_env: introduce  do_with_async_returning()

Similar to do_with_async(), but returning a non-void return type.
Will be used in test/perf.

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