Better default size for global index -> alias map

The alias -> (index -> alias) map, specifically the index -> alias one, typically just hold one entry, yet we eagerly initialize it to the number of indices. When there are many indices, each with many aliases, this is a very large overhead per alias...
closes elastic#6504

src/main/java/org/elasticsearch/cluster/metadata/MetaData.java

@@ -191,7 +191,7 @@ public static <T extends Custom> Custom.Factory<T> lookupFactorySafe(String type
                 AliasMetaData aliasMd = aliasCursor.value;
                 ImmutableOpenMap.Builder<String, AliasMetaData> indexAliasMap = (ImmutableOpenMap.Builder<String, AliasMetaData>) tmpAliases.get(aliasMd.alias());
                 if (indexAliasMap == null) {
-                    indexAliasMap = ImmutableOpenMap.builder(indices.size());
+                    indexAliasMap = ImmutableOpenMap.builder(1); // typically, there is 1 alias pointing to an index
                     tmpAliases.put(aliasMd.alias(), indexAliasMap);
                 }
                 indexAliasMap.put(index, aliasMd);