avx and ci16 supports for clients

[tingjunchen]

• Rename find_beacon_avx to find_beacon_avx2 and implement the avx version of find_beacon
• Add support for ci16 streaming for client streaming
• Add support for find_beacon_avx/avx2 to take input ref sequence in the ci16 format (or add support to enable correlation and peak finding directly using ci16 sequences). I think Agora supports client streaming in ci16 but the beacon detection has to first convert the ci_16 stream back to cf32 format.

[rdoosty]

I edited this and moved the LTS_SEQ things to a separate issue.

Also I think the ci_16 version would be a significant additional effort and it is not a priority, so I am inclined to move that to a separate issue as well. But I think avx should be relatively straightforward, as it uses pretty much the same algorithm.

[tingjunchen]

Agreed. Getting the avx version of find_beacon would be great.

Can we borrow the client-side ci16 streaming from Agora? Understand that the ci16 version of the correlator would be a significant effort

[rdoosty]

In Agora, we also use floating point for the correlation. The receiver, receives samples in ci16 but then we do a conversion to floating point before passing them on to find_beacon_avx: https://github.com/jianding17/Agora/blob/master/src/client/txrx_client.cpp#L524