Add a cache for fedora:digest property.

notes:

1) cache is declared as a static field; it's being called from a static method decorateContentNode(r,w). If only one instance of FedoraFileSystemConnector is created by modeshape, perhaps it makes sense to make it non-static. I haven't tried using FedoraFileSystemConnector yet to see how it's instantiated and how it's intended to be used.

2) cache is an LRU. Not sure if it makes sense in this context. I'm guessing decoreateContentNode gets called often for node? Also, the underlying ModeShape parent class (FileConnector) uses a method that seems to have some sort of caching mechanism in place. See o.m.jcr.api.getHexHash() javadoc comment for reference.

3) no tests for testing the cache

4) cache implementation itself is not inherently threadsafe. again depending on design, another implementation (leveldb ram cache) or custom cache could be used. Not sure if any Fcrepo code explicitly makes use of some cache.

 5) minor pom clean up is required.