Fix EZP-21988: async publishing should generate static cache

Static cache tasks weren't processed after publishing.

Asynchronous handling doesn't require that static cache
generation is left to the cronjob, since it will happen
after content has been published.

It will however slow publishing down, since publishing
processes will spend some time taking care of static
cache.
Merge pull request #842 from ezsystems/fix/EZP-21988-asynchronous_static_cache

Fix EZP-21988: async publishing should generate static cache