Get rid of cgo and dated libsodium-go wrapper

[martyanov]

It was probably covered by security review, but libsodium-go is dated and its README explicitly states that This code requires an independent audit check!.

Also, cgo is quite different runtime from gc and may be a problem to maintain, build, debug and base your project on.

I understand that the amount of work to be done may be huge. One option is to isolate cgo with libsodium into a separate service and provide RPC, just to limit cgo surface.

Just my 2 cents.