BN Fallback v2 (#2080)

## Issue Addressed

- Resolves #1883

## Proposed Changes

This follows on from @blacktemplar's work in #2018.

- Allows the VC to connect to multiple BN for redundancy.
  - Update the simulator so some nodes always need to rely on their fallback.
- Adds some extra deprecation warnings for `--eth1-endpoint`
- Pass `SignatureBytes` as a reference instead of by value.

## Additional Info

NA

Co-authored-by: blacktemplar <blacktemplar@a1.net>

Cargo.lock

@@ -7141,6 +7141,7 @@ dependencies = [
  "eth2_ssz",
  "eth2_ssz_derive",
  "exit-future",
+ "fallback",
  "futures 0.3.8",
  "hex",
  "hyper 0.13.9",