coord: Optimize read hold updates (MaterializeInc#14006)

This commit updates the logic for read holds updates so that an update
is only sent if the read timestamp has actually changed. This prevents
network calls from being sent that have no effect on the read policy.

src/adapter/src/coord/message_handler.rs

@@ -11,6 +11,7 @@
 //! messages from various sources (ex: controller, clients, background tasks, etc).
 
 use chrono::DurationRound;
+use timely::PartialOrder;
 use tracing::{event, Level};
 
 use mz_controller::{ComputeInstanceEvent, ControllerResponse};
@@ -314,7 +315,7 @@ impl<S: Append + 'static> Coordinator<S> {
             timeline,
             TimelineState {
                 mut oracle,
-                read_holds,
+                mut read_holds,
             },
         ) in global_timelines
         {
@@ -332,7 +333,9 @@ impl<S: Append + 'static> Coordinator<S> {
                 .fast_forward(now, |ts| self.catalog.persist_timestamp(&timeline, ts))
                 .await;
             let read_ts = oracle.read_ts();
-            let read_holds = self.update_read_hold(read_holds, read_ts).await;
+            if read_holds.time.less_than(&read_ts) {
+                read_holds = self.update_read_hold(read_holds, read_ts).await;
+            }
             self.global_timelines
                 .insert(timeline, TimelineState { oracle, read_holds });
         }