Notedb: Add a git repo based sequence implementation

This is intended to replace the next*Id() methods in ReviewDb. We
increment a counter stored in a blob on refs/sequences/*, and allow
multiple processes to reserve chunks of the number space so they can
operate concurrently. The resulting IDs are not quite monotonic, but
this is a common feature of distributed unique ID assignment; the
careful observer will note the same behavior on gerrit-review, for
example.

Change-Id: Ia5f59233df97b7eff71b1ec6af1d816975f28ad9

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/RefNames.java

@@ -46,6 +46,9 @@ public class RefNames {
   /** A change starred by a user */
   public static final String REFS_STARRED_CHANGES = "refs/starred-changes/";
 
+  /** Sequence counters in notedb. */
+  public static final String REFS_SEQUENCES = "refs/sequences/";
+
   /**
    * Prefix applied to merge commit base nodes.
    * <p>