Multichannel support for ConvolverNode

[orottier]

The current implementation only supports mono processing. See #537 for some discussion

[orottier]

Multi channel is now supported, however we need to add one bit still:

There is still a todo regarding input number of channel changes (i.e. from stereo to mono), I guess we need to add a bit of bookkeeping to properly handle the tail of the channel that disappeared, but not completely sure how to do that yet.