brontide: switch to using libsecp256k1's public ECDH API
brontide: switch to using libsecp256k1's public ECDH API

This commit modifies the opening brontide handshake to use
libsecp256k1's public ECDH API throughout the handshake rather than the
current method which just returns the x-coordinate of the generated
point.

This change was made in order to align the current spec draft with the
aforementioned library since it’s very popular within the pace and
strives to only expose safe API’s to end users.

AndrewSamokhvalov/lnd,bryanvu/lnd,aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,ddustin/lnd,bryanvu/lnd,ddustin/lnd