Support for locks

[Sleavely]

If using a state store such as a database, multiple users may be trying to perform migrations at the same time. This would yield unwanted results.

Suggested sample configuration:

{
  /**
   * Invoked at the beginning of a migration round to establish a global lock.
   * If `lockTimeout` has been specified, this method will be wrapped
   * in Promise.race() against a timer that causes the migration to fail.
   *
   * @param context
   * @return Once a lock has been obtained.
   */
  lock: async (context) => {},

  /**
   * Invoked to release any global lock acquired by the `lock` function.
   */
  unlock: async (context) => {},

  /**
   * The number of milliseconds to give up after if a lock cannot be obtained
   * or released. This is only applicable if the `lock` function is implemented.
   */
  lockTimeout: 0,
}

[Sleavely]

This is one of the features that exist in migrat but I'm not sure whether it would be overdoing it or if it's useful. Marking as discussion for now.