Add possibility to destroy (disconnect)

[fedyk]

Currently, after creating an instance of MTProto, there are no easy way to destroy instance (close open ws/socket connections and remove listeners).

Here I added a destroy method. The usage is following:

// 1. Create instance
const mtproto = new MTProto({ api_id, api_hash, storageOptions: {...}});

// 2. Call any method(-s)
mtproto.call('help.getNearestDc').then(result => ...});

// 3. Destroy: close open connections, reject pending requests, cleanup event's listeners
mtproto.destroy()

[abdulgalimov]

Please tell me, can these edits be merge to the master? Need a destroy method. Thanks

[Alexxosipov]

@alik0211 can you please merge it?