Only process incoming audio when listener attached
Only process incoming audio when listener attached

Only process incoming udpClient messages when a listener is attached to the 'incoming' event of the stream. This will address a possible performance issue when an audio context is obtained from the unnecessary creation of expensive objects.

This is achieved by only attaching the message handler for the udpClient when a listener is attached to the 'incoming' event of the stream.

It is no longer necessary to manually set whether or not to handle incoming audio; thus the option to do so is therefore removed.

These changes do not make any breaking changes to the API.
izy521/discord.io,zaza7/discord.io,izy521/node-discord