Acquire increments RefCount.

Lookup doesn't update RefCount.

Release decrements RefCount.

CompatPlugin is an abstraction to handle both v2(new) and v1(legacy) plugins.

CountedPlugin is a plugin which is reference counted.

PluginAddr is a plugin that exposes the socket address for creating custom clients rather than the built-in `*plugins.Client`.

PluginGetter is the interface implemented by Store.

PluginWithV1Client is a plugin that directly utilizes the v1/http plugin client.