Eviction with strategy but no maxEntries does not make sense

A configuration like this can be confusing cos no maxEntries is set, and without it there's no trigger to start evicting stuff:

<default>
<eviction strategy="FIFO" wakeUpInterval="6000" /><!-- 6 seconds -->
</default>

So, we should come up with some WARN/ERROR message indicating that maxEntries is missing unless NONE strategy is used.