Elasticsearch: Encapsulate supported versions in an enum

Supported versions are encapsulated in a new enum, ElasticVersion, that
provides a method to translate from the version string returned by the
Elasticsearch server.

Translation of the version works on prefix matching, with granularity
at the X.Y level. For example the version strings "2.4.0" and "2.4.6"
will both resolve to the V2_4 enum value.

This assumes that Elasticsearch has backwards compatibility between all
minor versions in the X.Y series, i.e. "2.4.0" and "2.4.6" should both
be equally compatible. If this turns out to not be the case, further
support can be added later.

ElasticVersionManager is renamed to ElasticIndexVersionManager to avoid
confusion with the ElasticVersion enum.

Change-Id: I4dfe443f0e5d26652433334cef6be8c2727e9317

gerrit-elasticsearch/src/test/java/com/google/gerrit/elasticsearch/ElasticVersionTest.java

@@ -0,0 +1,45 @@
+// Copyright (C) 2018 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.elasticsearch;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+public class ElasticVersionTest {
+  @Rule public ExpectedException exception = ExpectedException.none();
+
+  @Test
+  public void supportedVersion() throws Exception {
+    assertThat(ElasticVersion.forVersion("2.4.0")).isEqualTo(ElasticVersion.V2_4);
+    assertThat(ElasticVersion.forVersion("2.4.6")).isEqualTo(ElasticVersion.V2_4);
+
+    assertThat(ElasticVersion.forVersion("5.6.0")).isEqualTo(ElasticVersion.V5_6);
+    assertThat(ElasticVersion.forVersion("5.6.9")).isEqualTo(ElasticVersion.V5_6);
+
+    assertThat(ElasticVersion.forVersion("6.2.0")).isEqualTo(ElasticVersion.V6_2);
+    assertThat(ElasticVersion.forVersion("6.2.4")).isEqualTo(ElasticVersion.V6_2);
+  }
+
+  @Test
+  public void unsupportedVersion() throws Exception {
+    exception.expect(ElasticVersion.InvalidVersion.class);
+    exception.expectMessage(
+        "Invalid version: [4.0.0]. Supported versions: " + ElasticVersion.supportedVersions());
+    ElasticVersion.forVersion("4.0.0");
+  }
+}