SE: No new fibers after exhausted plan

Execution plan is shared between fibers. That means if one fiber
declares the plan was exhausted, any new spawned fiber will do the same.
Because of that, there is no need to spawn new fibers in this situation.
The way to do that is setting remaining speculative retries to 0.
We can't return immediately - we still have to wait for remaining
running fibers.

Author: Lorak-mmk

scylla/src/policies/speculative_execution.rs

@@ -180,6 +180,11 @@ where
                     } else {
                         last_error = Some(r)
                     }
+                } else {
+                    // The only case where None is returned is when execution plan was exhausted.
+                    // If so, there is no reason to start any more fibers.
+                    // We can't always return - there may still be fibers running.
+                    retries_remaining = 0;
                 }
                 if async_tasks.is_empty() && retries_remaining == 0 {
                     return last_error.unwrap_or({