reactor: update lowres_clock when max_task_backlog is exceeded

lowres_clock promises task_quota resolution, but we forgot to update
lowres_clock when first, there's a need to preempt *and* second,
max_task_backlog is exceeded. all tasks that follow until backlog
reduces below max target, will find lowres_clock with "now" time as
of beginning of run_tasks() execution, despite multiple task quotas
may have passed since then.

Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>

src/core/reactor.cc

@@ -2701,6 +2701,7 @@ void reactor::run_tasks(task_queue& tq) {
                 // need_preempt() checks breaking out of loops and .then() calls. See
                 // #302.
                 reset_preemption_monitor();
+                lowres_clock::update();
             }
         }
     }