Plugin version negotiation (#5434)

* Plugin version updates

* Update datatbase plugins

* Revert netRPC deletions

* Revert netRPC deletions

* Update plugins to serve both versions

* Update database plugins

* Add Initialize back in

* revert pointer changes

* Add deprecation warning

* Update tests

* Update go-plugin

* Review Feedback

builtin/logical/database/dbplugin/plugin_test.go

@@ -127,6 +127,7 @@ func TestPlugin_NetRPC_Main(t *testing.T) {
 		return
 	}
 
+	os.Unsetenv(pluginutil.PluginVaultVersionEnv)
 	p := &mockPlugin{
 		users: make(map[string][]string),
 	}