Network protocol upgrades (#2345)

This provides a number of upgrades to gossipsub and discovery. 

The updates are extensive and this needs thorough testing.

Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.50.0 AS builder
+FROM rust:1.52.1 AS builder
 RUN apt-get update && apt-get install -y cmake
 COPY . lighthouse
 ARG PORTABLE