Add destroy method to Plugin

[beam-sysuser]

Add destroy() method to Plugin that is called on all plugins before Gyro shuts down, the default implementation should do nothing.

This is needed to make Retrofit + OkHTTP work correctly. OkHTTP creates a background thread that never exits on its own so we need to force a cleanup when Gyro is about to shutdown. There is currently no way to do this.

[GYRO-146] created by beetlebugorg