This makes two main changes to the plugin system:
* Instead of assuming that each plugin will have exactly one type,
inspect each plugin to see which interfaces it provides, and
register it as a provider of each provided interface. This can
allow a single .so file to provide multiple interfaces, which
will likely be necessary for aggregating certain types of info.
* Rather than using dependency injection and having to propagate
the plugin system all throughout Geth, have a default plugin
loader so we need only import the module and make calls to it.
If the plan were to integrate this into mainline Geth, I would
say we use dependency injection and take the time to pass the
plugin loader throughout the codebase, but as I expect this to
be a fork that has to pull upstream changes, this approach
should make merge conflicts much less common.
