new pool api

[BillyDM]

• The "sampler pool" has been reworked into an AudioNodePool that is both generic over the audio backend and generic over the type of starting node. This allows users to create a pool with whatever custom node they want, not just the sampler node.
• The methods in the AudioNodePool now accept an optional time parameter for scheduling.
• The user can now get the state of the starting node directly from an AudioNodePool.
• TimedContextQueue has been removed. Instead, ContextQueue just stores the time optionally if the scheduled_events feature is enabled.