support for concurrent instances of the same flow

This would allow using the same flow several times in different browser tabs/windows, see #99. /cc @Jaspur

It introduces a randomly generated so-called "instance id", which is then used as part of the storage key. It's passed as a hidden field from step to step, and is added to all dsn (dynamic step navigation) links by the Twig extension.

Adding this feature causes a BC break, as just performing a GET request would now result in a new flow instance, which was not the case before with dsn enabled. So I'll target version 3.0 for this one.