Replace mapDebugger in StatsDebugger with flatMapDebugger

The only method that has a return value in the StatsDebugger interface
returns an Optional, so this makes more sense.

Author: hazefully

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/query/plan/cascades/CascadesPlanner.java

@@ -332,7 +332,7 @@ public QueryPlanResult planQuery(@Nonnull final RecordQuery query,
                 .put(QueryPlanInfoKeys.MAX_TASK_QUEUE_SIZE, maxQueueSize)
                 .put(QueryPlanInfoKeys.CONSTRAINTS, constraints)
                 .put(QueryPlanInfoKeys.STATS_MAPS,
-                        StatsDebugger.getDebuggerMaybe().flatMap(StatsDebugger::getStatsMaps)
+                        StatsDebugger.flatMapDebugger(StatsDebugger::getStatsMaps)
                                 .orElse(null))
                 .build();
         return new QueryPlanResult(plan, info);
@@ -376,7 +376,7 @@ public QueryPlanResult planGraph(@Nonnull final Supplier<Reference> referenceSup
                     QueryPlanInfo.newBuilder()
                             .put(QueryPlanInfoKeys.CONSTRAINTS, constraints)
                             .put(QueryPlanInfoKeys.STATS_MAPS,
-                                    StatsDebugger.getDebuggerMaybe().flatMap(StatsDebugger::getStatsMaps)
+                                    StatsDebugger.flatMapDebugger(StatsDebugger::getStatsMaps)
                                             .orElse(null))
                             .build());
         } finally {

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/query/plan/cascades/debug/StatsDebugger.java

@@ -50,8 +50,8 @@ static void withDebugger(@Nonnull final Consumer<StatsDebugger> action) {
     }
 
     @Nonnull
-    static <T> Optional<T> mapDebugger(@Nonnull final Function<StatsDebugger, T> function) {
-        return getDebuggerMaybe().map(function);
+    static <T> Optional<T> flatMapDebugger(@Nonnull final Function<StatsDebugger, Optional<T>> function) {
+        return getDebuggerMaybe().flatMap(function);
     }
 
     @Nonnull