ConvolverNode improvements

[orottier]

• multi channel support
• use non-uniform blocks for the Frequency Delay Lines to improve performance - http://ericbattenberg.com/school/partconvDAFx2011.pdf
• run the larger FFT blocks on a separate thread

[b-ma]

Hey,

I just found this: https://docs.rs/fft-convolver/0.1.0/fft_convolver/, maybe we could have a shot?

[orottier]

Interesting, we could have a try. But at first glance it looks a lot like our implementation

• fixed block size frequency delay lines
• no multi-channel support
• no threadpool support

Would still be interesting to do a performance check

[orottier]

Closing after performance has improved in #537 , tracking multi channel in #542