refactor(object-id): provide static forms for OID generation

This appears to have been an oversight during the 4.x refactoring.
These methods should be statics, not called on instances of an
ObjectId class. Moved them over to statics, provided a non-BC
breaking instance method, and documented them as deprecated.

Author: mbroadst

lib/objectid.js

@@ -138,6 +138,13 @@ class ObjectId {
    * @return {number} returns next index value.
    * @ignore
    */
+  static get_inc() {
+    return (ObjectId.index = (ObjectId.index + 1) % 0xffffff);
+  }
+
+  /**
+   * @deprecated Please use the static form instead
+   */
   get_inc() {
     return (ObjectId.index = (ObjectId.index + 1) % 0xffffff);
   }
@@ -149,6 +156,13 @@ class ObjectId {
    * @return {number} returns next index value.
    * @ignore
    */
+  static getInc() {
+    return ObjectId.get_inc();
+  }
+
+  /**
+   * @deprecated Please use the static form instead
+   */
   getInc() {
     return this.get_inc();
   }
@@ -160,7 +174,7 @@ class ObjectId {
    * @param {number} [time] optional parameter allowing to pass in a second based timestamp.
    * @return {Buffer} return the 12 byte id buffer string.
    */
-  generate(time) {
+  static generate(time) {
     if ('number' !== typeof time) {
       time = ~~(Date.now() / 1000);
     }
@@ -189,6 +203,13 @@ class ObjectId {
     return buffer;
   }
 
+  /**
+   * @deprecated Please use the static form instead
+   */
+  generate(time) {
+    return ObjectId.generate(time);
+  }
+
   /**
    * Converts the id into a 24 byte hex string for printing
    *