Make TURN configuration dynamic

Also enables conditionally including more TURN servers for gradual
rollouts

service/src/test/java/org/whispersystems/textsecuregcm/util/WeightedRandomSelectTest.java

@@ -0,0 +1,46 @@
+package org.whispersystems.textsecuregcm.util;
+
+import org.junit.jupiter.api.Test;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class WeightedRandomSelectTest {
+
+  @Test
+  public void test5050() {
+    final WeightedRandomSelect<String> selector = new WeightedRandomSelect<>(
+        List.of(new Pair<>("a", 1L), new Pair<>("b", 1L)));
+    final Map<String, Long> counts = Stream.generate(selector::select)
+        .limit(1000)
+        .collect(Collectors.groupingBy(s -> s, Collectors.counting()));
+    assertThat(counts.get("a")).isGreaterThan(1);
+    assertThat(counts.get("b")).isGreaterThan(1);
+  }
+
+  @Test
+  public void testAlways() {
+    final WeightedRandomSelect<String> selector = new WeightedRandomSelect<>(
+        List.of(new Pair<>("a", 1L), new Pair<>("b", 0L)));
+    final Map<String, Long> counts = Stream.generate(selector::select)
+        .limit(1000)
+        .collect(Collectors.groupingBy(s -> s, Collectors.counting()));
+    assertThat(counts.get("a")).isEqualTo(1000);
+    assertThat(counts).doesNotContainKey("b");
+  }
+
+  @Test
+  public void testThree() {
+    final WeightedRandomSelect<String> selector = new WeightedRandomSelect<>(
+        List.of(new Pair<>("a", 33L), new Pair<>("b", 33L), new Pair<>("c", 33L)));
+    final Map<String, Long> counts = Stream.generate(selector::select)
+        .limit(1000)
+        .collect(Collectors.groupingBy(s -> s, Collectors.counting()));
+    assertThat(counts.get("a")).isGreaterThan(1);
+    assertThat(counts.get("b")).isGreaterThan(1);
+    assertThat(counts.get("c")).isGreaterThan(1);
+  }
+}