Add a way to configure which plugins to use

[sh00s]

Currently if multiple plugins are loaded of the same type (aside from built-in plugins and plugins that allow multiple active instances of the same types - e.g. ConfigSources), PluginRegistry will fail, as it doesn't have a way to decide which one of those to use.

Instead it should be configurable (via settings or via the configuration file) which plugin to use for each plugin type. Then PluginRegistry::setCurrentSinglePlugins() should use that information to pick the active plugin:
https://github.com/facebookincubator/LogDevice/blob/e6424a70735b589d0be1ed8a25f7a273a2198df3/logdevice/common/plugin/PluginRegistry.cpp#L85-L88

It should also subscribe to config/setting changes so that you can reconfigure which plugin to use on the fly.