replace soxr for tempo stretching with something that does not change pitch

[christf]

Alternatives are:

• librubberband which can only work with 2048 bytes at a time
• tempo effect from libsox - from a casual glance it seems it can work with arbitrary input sizes

This will require more analysis.

[christf]

Having looked at this again, there are a few gotchas:

• librubberband wil work only on float. As such the client will have to convert to float, then apply librubberband and then feed alsa. There are two ways of doing this: always convert to float and open the alsa device in an appropriate way or only convert (back) when stretching is required. The latter seems to be the better choice. However librubberband wants to be fed samples of 2048 bytes at a time. This will not alighn with the chunk size of snapcastc. Worst case 2047 bytes per chunk are left unstretched.

• libsox is available to do tempo stretching however it only works on files, not on byte arrays in memory. There are a few threads from 2013 where Ulrich states that streaming support should be added to sox but I was unable to find it in the docs yet.