improves efficiency of migration set clean up (#4474)

Cleanup of the set of migrating tablets would read the entire metadata
table and read all tablet extents into memory. This changes limits the
scan to the tablets section of the metadata table and avoids reading all
tablet extents into memory.

server/manager/src/main/java/org/apache/accumulo/manager/Manager.java

@@ -95,6 +95,7 @@
 import org.apache.accumulo.core.metadata.TabletLocationState;
 import org.apache.accumulo.core.metadata.TabletState;
 import org.apache.accumulo.core.metadata.schema.Ample.DataLevel;
+import org.apache.accumulo.core.metadata.schema.MetadataSchema;
 import org.apache.accumulo.core.metadata.schema.MetadataSchema.TabletsSection.TabletColumnFamily;
 import org.apache.accumulo.core.metrics.MetricsUtil;
 import org.apache.accumulo.core.replication.thrift.ReplicationCoordinator;
@@ -710,14 +711,17 @@ private void cleanupNonexistentMigrations(final AccumuloClient accumuloClient)
         throws TableNotFoundException {
       Scanner scanner = accumuloClient.createScanner(MetadataTable.NAME, Authorizations.EMPTY);
       TabletColumnFamily.PREV_ROW_COLUMN.fetch(scanner);
-      Set<KeyExtent> found = new HashSet<>();
+      scanner.setRange(MetadataSchema.TabletsSection.getRange());
+      Set<KeyExtent> notSeen;
+      synchronized (migrations) {
+        notSeen = new HashSet<>(migrations.keySet());
+      }
       for (Entry<Key,Value> entry : scanner) {
         KeyExtent extent = KeyExtent.fromMetaPrevRow(entry);
-        if (migrations.containsKey(extent)) {
-          found.add(extent);
-        }
+        notSeen.remove(extent);
       }
-      migrations.keySet().retainAll(found);
+      // remove tablets that used to be in migrations and were not seen in the metadata table
+      migrations.keySet().removeAll(notSeen);
     }
 
     /**