Refactor Node from interface to value type

The modeling of BootstrapNode as an enum implementing the Node interface
became awkward, requiring an ugly, package-private 'NodeImpl' class and
other problems.

This change eliminates NodeImpl, refactors Node from being an interface
to being a value type [1,2], and refactors BootstrapNode from being an
enum to being an interface (the #all method there takes the place of
what was the enum's inherited #values method). This is slightly more
verbose in the end than being modeled as an enum, but in the end, we
were never using BootstrapNode as an enum (e.g., never used in switch
statements or == equality comparisons, etc).

[1]: http://blog.joda.org/2014/03/valjos-value-java-objects.html
[2]: http://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html
Rename i.b.network.{BootstrapNode=>BootstrapNodes}