Make TURN configuration dynamic

Also enables conditionally including more TURN servers for gradual
rollouts

service/src/main/java/org/whispersystems/textsecuregcm/util/WeightedRandomSelect.java

@@ -0,0 +1,54 @@
+package org.whispersystems.textsecuregcm.util;
+
+import java.util.List;
+import java.util.concurrent.ThreadLocalRandom;
+
+/**
+ * Select a random item according to its weight
+ *
+ * @param <T> the type of the objects to select from
+ */
+public class WeightedRandomSelect<T> {
+
+  List<Pair<T, Long>> weightedItems;
+  long totalWeight;
+
+  public WeightedRandomSelect(List<Pair<T, Long>> weightedItems) throws IllegalArgumentException {
+    this.weightedItems = weightedItems;
+    this.totalWeight = weightedItems.stream().mapToLong(Pair::second).sum();
+
+    weightedItems.stream().map(Pair::second).filter(w -> w < 0).findFirst().ifPresent(invalid -> {
+      throw new IllegalArgumentException("Illegal selection weight " + invalid);
+    });
+
+    if (weightedItems.isEmpty() || totalWeight == 0) {
+      throw new IllegalArgumentException("Cannot create an empty weighted random selector");
+    }
+  }
+
+  public T select() {
+    if (weightedItems.size() == 1) {
+      return weightedItems.get(0).first();
+    }
+    long select = ThreadLocalRandom.current().nextLong(0, totalWeight);
+    long current = 0;
+    for (Pair<T, Long> item : weightedItems) {
+      /*
+        Accumulate weights for each item and select the first item whose
+        cumulative weight exceeds the selected value. nextLong() is exclusive,
+        so by the last item we're guaranteed to find a value as the
+        last item's weight is one more than the maximum value of select.
+      */
+      current += item.second();
+      if (current > select) {
+        return item.first();
+      }
+    }
+    throw new IllegalStateException("totalWeight " + totalWeight + " exceeds item weights");
+  }
+
+  public static <T> T select(List<Pair<T, Long>> weightedItems) {
+    return new WeightedRandomSelect<T>(weightedItems).select();
+  }
+
+}