Refactor _getMilestoneSnapshotByTimestamp

This change refactors the `_getMilestoneSnapshotByTimestamp` method
according to code review comments.

lib/mongo-milestone-db.js

@@ -111,20 +111,27 @@ class MongoMilestoneDB extends MilestoneDB {
       return Promise.reject(new InvalidTimestampError());
     }
 
-    const nullSortOrder = isAfterTimestamp ? -1 : 1;
+    const limit = 1;
+    let query;
+    let sortOrder;
+
+    if (timestamp === null) {
+      query = {d: id};
+      sortOrder = isAfterTimestamp ? -1 : 1;
+    } else {
+      const comparator = isAfterTimestamp ? {$gte: timestamp} : {$lte: timestamp};
+      query = {
+        d: id,
+        'm.mtime': comparator,
+      };
+      sortOrder = isAfterTimestamp ? 1 : -1;
+    }
 
-    const query = {d: id};
     const options = {
-      limit: 1,
-      sort: {'m.mtime': nullSortOrder},
+      limit: limit,
+      sort: {'m.mtime': sortOrder},
     };
 
-    if (timestamp !== null) {
-      const comparator = isAfterTimestamp ? {$gte: timestamp} : {$lte: timestamp};
-      query['m.mtime'] = comparator;
-      options.sort['m.mtime'] = -nullSortOrder;
-    }
-
     return this._getMilestoneSnapshotByQuery(collectionName, query, options);
   }