[PSR-6] Added "flag" function for stampede protection

This pull request is in response to a discussion on the mailing list: https://groups.google.com/forum/#!topic/php-fig/WYShSmYYk9M

The main idea is that a "flag" function would allow implementing libraries to respond to write operations intelligently and take measures to prevent the "stampede effect"/"dog pile" problem that can occur when a cache miss happens for an expensive item.