JENA-901 Use Guava for cache, shadowed in jena-core

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In comparison with #47, this shadows Guava directly in jena-core without introducing jena-shadowed-ext.

This is thus acceptable as a patch-version update - but at the downside that jena-core have to do

import com.google.common.cache.Cache

directly - while all other Jena modules would have to use

import org.apache.jena.impl.ext.com.google.common.cache.Cache

as it is shadowed with the JAR. This will cause problems in the other modules in an Eclipse-like environment, as the IDE will re-expose com.google version if jena-core is open - and they would then seemingly not be able to find the oaj.impl.ext.* version as that only exist in the Maven-built JAR.

In this scenario, it would probably be cleaner for the other modules to also shadow guava - in addition to the space-wastage, this probably becomes a bit awkward - it also means you can't pass any Guava structures between Jena modules.

One advantage however is that if they all shadow individually, then they can shadow with the minimizeJar option to only get the few Guava classes used (e.g. in jena-core only the Cache).

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I am closing this as the feedback on guava shadowed in jena-core was negative (and I agree!)