feat: add concept of data-bearing type to ServerDescription

Author: mbroadst

lib/core/sdam/server_description.js

@@ -19,6 +19,13 @@ const WRITABLE_SERVER_TYPES = new Set([
   ServerType.Mongos
 ]);
 
+const DATA_BEARING_SERVER_TYPES = new Set([
+  ServerType.RSPrimary,
+  ServerType.RSSecondary,
+  ServerType.Mongos,
+  ServerType.Standalone
+]);
+
 const ISMASTER_FIELDS = [
   'minWireVersion',
   'maxWireVersion',
@@ -99,6 +106,13 @@ class ServerDescription {
     return this.type === ServerType.RSSecondary || this.isWritable;
   }
 
+  /**
+   * @return {Boolean} Is this server data bearing
+   */
+  get isDataBearing() {
+    return DATA_BEARING_SERVER_TYPES.has(this.type);
+  }
+
   /**
    * @return {Boolean} Is this server available for writes
    */