Preload write is performed on persistence thread

With ISPN-9722 preload was made non blocking. However the writes of the entries to memory are performed on the persistence thread pool, which is incorrect. We should instead do this on async (CPU) thread pool.

This can cause issues with preload with passivation enabled as seen by this stack trace https://paste.fedoraproject.org/paste/Dn~uOsVL1EI7Iq5EK1kIzA