scheduled eviction and graceful shutdown

[muhamadazmy]

• handle signal and shutdown gracefully (later we can do clean up)
• send an evict control message to device every 500ms
• the device can then ignore the msg if the device is under heavy load it only process it if the device have been ideal for over 500ms
• the eviction process should try the best it can do in only 50ms otherwise wait another 500ms before it get another eviction request

This will make the eviction happen continuesly in the background without affecting IO speed much

The device can still do eviction during an IO operation if needed to be able to load a page from persisted (slow) storage.

The device can decide to ignore the scheduled eviction if the device is under heavy io operations.