Sequences: Introduce constants for configuration keys/values

Use the existing constants for "accounts" and "changes", and add
new constants for "noteDb" and "sequenceBatchSize".

Add new constants for the default sequence batch sizes for accounts
and changes.

Change-Id: I6ed0c09bcbf4fc636fcc6a0107b75f971fb2d080
diff --git a/java/com/google/gerrit/server/notedb/Sequences.java b/java/com/google/gerrit/server/notedb/Sequences.java
index 9e8c541..d4144ce 100644
--- a/java/com/google/gerrit/server/notedb/Sequences.java
+++ b/java/com/google/gerrit/server/notedb/Sequences.java
@@ -31,6 +31,11 @@
 
 @Singleton
 public class Sequences {
+  private static final String SECTION_NOTEDB = "noteDb";
+  private static final String KEY_SEQUENCE_BATCH_SIZE = "sequenceBatchSize";
+  private static final int DEFAULT_ACCOUNTS_SEQUENCE_BATCH_SIZE = 1;
+  private static final int DEFAULT_CHANGES_SEQUENCE_BATCH_SIZE = 20;
+
   public static final String NAME_ACCOUNTS = "accounts";
   public static final String NAME_GROUPS = "groups";
   public static final String NAME_CHANGES = "changes";
@@ -59,7 +64,12 @@
       AllUsersName allUsers,
       MetricMaker metrics) {
 
-    int accountBatchSize = cfg.getInt("noteDb", "accounts", "sequenceBatchSize", 1);
+    int accountBatchSize =
+        cfg.getInt(
+            SECTION_NOTEDB,
+            NAME_ACCOUNTS,
+            KEY_SEQUENCE_BATCH_SIZE,
+            DEFAULT_ACCOUNTS_SEQUENCE_BATCH_SIZE);
     accountSeq =
         new RepoSequence(
             repoManager,
@@ -69,7 +79,12 @@
             () -> FIRST_ACCOUNT_ID,
             accountBatchSize);
 
-    int changeBatchSize = cfg.getInt("noteDb", "changes", "sequenceBatchSize", 20);
+    int changeBatchSize =
+        cfg.getInt(
+            SECTION_NOTEDB,
+            NAME_CHANGES,
+            KEY_SEQUENCE_BATCH_SIZE,
+            DEFAULT_CHANGES_SEQUENCE_BATCH_SIZE);
     changeSeq =
         new RepoSequence(
             repoManager,