Add support for unscheduled write-behind cache stores to Infinispan subsytem

Infinispan subsystem should support unscheduled write-behind cache stores. Currently it is not possible to set this up.

In Infinispan this is done via <async> sub-element of <loader> element. E.g. <async enabled="true" threadPoolSize="10" />

Documentation for EDG is mentioning both unscheduled and scheduled write-behind cache stores even though scheduled one is not implemented even in Infinispan (ISPN-328). OTOH, the unscheduled one is implemented and should be available in EDG.

So take into account that in future there might be also scheduled write-behind store available and infinispan schema for JBossAS should count with that.