Added argument to drain that provides the list of packets that are drained

[bytenik]

The kind of change this PR does introduce

• [ ] a bug fix
• [x] a new feature
• [ ] an update to the documentation
• [ ] a code change that improves performance
• [ ] other

Current behavior

'drain' event occurs with no arguments

New behavior

'drain' event occurs with an argument containing the list of packets that were sent by the transport

Other information (e.g. related issues)