[router] Made most VeniceRouterConfig fields final

There are only two exceptions, related to throttling, which are allowed
to be mutated post-init.

Miscellaneous:

- Deleted a dead config: router.compute.fast.avro.enabled
- Made VeniceProperties::getMap return an unmodifiable map.

internal/venice-client-common/src/main/java/com/linkedin/venice/utils/VeniceProperties.java

@@ -427,26 +427,22 @@ public Map<String, String> getMap(String key) {
     if (!containsKey(key)) {
       throw new UndefinedPropertyException(key);
     }
-    Map<String, String> map = new HashMap<>();
     List<String> keyValuePairs = this.getList(key);
-
-    for (String pair: keyValuePairs) {
-      Pair<String, String> keyValuePair = splitAnEntryToKeyValuePair(pair);
-      map.put(keyValuePair.getFirst(), keyValuePair.getSecond());
-    }
-    return map;
-  }
-
-  private Pair<String, String> splitAnEntryToKeyValuePair(String entry) {
-    if (!entry.contains(":")) {
-      throw new VeniceException("Invalid config. Expect each entry to contain at least one \":\". Got: " + entry);
+    Map<String, String> map = new HashMap<>(keyValuePairs.size());
+
+    for (String keyValuePair: keyValuePairs) {
+      // One entry could have multiple ":". For example, "<ID>:<Kafka URL>:<port>". In this case, we split the String by
+      // its first ":" so that we get key=<ID> and value=<Kafka URL>:<port>
+      int indexOfFirstColon = keyValuePair.indexOf(':');
+      if (indexOfFirstColon == -1) {
+        throw new VeniceException(
+            "Invalid config. Expect each entry to contain at least one \":\". Got: " + keyValuePair);
+      }
+      String mapKey = keyValuePair.substring(0, indexOfFirstColon);
+      String mapValue = keyValuePair.substring(indexOfFirstColon + 1);
+      map.put(mapKey, mapValue);
     }
-    // One entry could have multiple ":". For example, "<ID>:<Kafka URL>:<port>". In this case, we split the String by
-    // its first ":" so that we get key=<ID> and value=<Kafka URL>:<port>
-    int indexOfFirstColon = entry.indexOf(':');
-    String key = entry.substring(0, indexOfFirstColon);
-    String value = entry.substring(indexOfFirstColon + 1);
-    return new Pair<>(key, value);
+    return Collections.unmodifiableMap(map);
   }
 
   public Properties toProperties() {

internal/venice-client-common/src/test/java/com/linkedin/venice/utils/VenicePropertiesTest.java

@@ -1,9 +1,10 @@
 package com.linkedin.venice.utils;
 
 import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertThrows;
 
+import com.linkedin.venice.exceptions.VeniceException;
 import java.util.Map;
-import java.util.Properties;
 import org.testng.Assert;
 import org.testng.annotations.Test;
 
@@ -22,12 +23,21 @@ public void testConvertSizeFromLiteral() {
 
   @Test
   public void testGetMapWhenMapIsStringEncoded() {
-    Properties properties = new Properties();
-    properties.put("region.to.pubsub.broker.map", "prod:https://prod-broker:1234,dev:dev-broker:9876");
-    VeniceProperties veniceProperties = new VeniceProperties(properties);
+    VeniceProperties veniceProperties =
+        new PropertyBuilder().put("region.to.pubsub.broker.map", "prod:https://prod-broker:1234,dev:dev-broker:9876")
+            .build();
     Map<String, String> map = veniceProperties.getMap("region.to.pubsub.broker.map");
     assertEquals(map.size(), 2);
     assertEquals(map.get("prod"), "https://prod-broker:1234");
     assertEquals(map.get("dev"), "dev-broker:9876");
+
+    // Map should be immutable
+    assertThrows(() -> map.put("foo", "bar"));
+
+    // Invalid encoding
+    VeniceProperties invalidVeniceProperties =
+        new PropertyBuilder().put("region.to.pubsub.broker.map", "prod:https://prod-broker:1234,dev;dev-broker")
+            .build();
+    assertThrows(VeniceException.class, () -> invalidVeniceProperties.getMap("region.to.pubsub.broker.map"));
   }
 }

internal/venice-common/src/main/java/com/linkedin/venice/ConfigKeys.java

@@ -1050,11 +1050,6 @@ private ConfigKeys() {
 
   public static final String ROUTER_CLIENT_DECOMPRESSION_ENABLED = "router.client.decompression.enabled";
 
-  /**
-   * Whether to enable fast-avro in router;
-   */
-  public static final String ROUTER_COMPUTE_FAST_AVRO_ENABLED = "router.compute.fast.avro.enabled";
-
   /**
    * Socket timeout config for the connection manager from router to server
    */