fixed loading of missing lazy blocks

Per this conversation https://twitter.com/GordonLesti/status/532450489397706752
the real bug was that the lazy blocks session flag wasn't updated when a new lazy block was being added from the admin. 

For example, if my session started with the default FPC config, and then I edit the fpc settings, adding new lazy blocks, those new blocks would not show up on the frontend, because the "session lock flag" would not change.