This is a first cut at giving NodeInfo the ability to carry a flexible
list of heterogeneous info responses. The trick is to be able to
serialize and deserialize an arbitrary list of blocks of information. It
is convenient to be able to deserialize into usable Java objects so that
we can aggregate nodes stats for the cluster stats endpoint.
In order to provide a little bit of clarity about which objects can and
can't be used as info blocks, I've introduced a new interface called
"ReportingService."
I have removed the hard-coded getters (e.g., getOs()) in favor of a
flexible method that can return heterogeneous kinds of info blocks
(e.g., getInfo(OsInfo.class)). Taking a class as an argument removes the
need to cast in the client code.
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