Update references to ReviewDb in comments

This changed considered all instances of "ReviewDb" in .java files. Many
of the comments changed, but there are exceptions:
 * Comments referencing old ReviewDb behavior in order to explain why
   things work a certain way in NoteDb.
 * Some TODO comments suggesting that we do something different after
   removing ReviewDb. Eliminating these would require actual code
   changes.

Change-Id: I18c0ac4038ae98aa3b9eabe4e68e23b6516ea3e8

java/com/google/gerrit/server/schema/SchemaCreatorImpl.java

@@ -98,7 +98,8 @@ public class SchemaCreatorImpl implements SchemaCreator {
     // We have to create the All-Users repository before we can use it to store the groups in it.
     allUsersCreator.setAdministrators(admins).create();
 
-    // Don't rely on injection to construct Sequences, as it requires ReviewDb.
+    // Don't rely on injection to construct Sequences, as the default GitReferenceUpdated has a
+    // thick dependency stack which may not all be available at schema creation time.
     Sequences seqs =
         new Sequences(
             config,