Weak subjectivity start from genesis (#1675)

## Issue Addressed
Solution 2 proposed here: https://github.com/sigp/lighthouse/issues/1435#issuecomment-692317639

## Proposed Changes
- Adds an optional `--wss-checkpoint` flag that takes a string `root:epoch`
- Verify that the given checkpoint exists in the chain, or that the the chain syncs through this checkpoint. If not, shutdown and prompt the user to purge state before restarting.

## Additional Info


Co-authored-by: Paul Hauner <paul@paulhauner.com>

Cargo.lock

@@ -330,6 +330,7 @@ dependencies = [
  "eth2_ssz",
  "eth2_ssz_derive",
  "eth2_ssz_types",
+ "exit-future",
  "fork_choice",
  "futures 0.3.5",
  "genesis",
@@ -386,6 +387,7 @@ dependencies = [
  "exit-future",
  "futures 0.3.5",
  "genesis",
+ "hex 0.4.2",
  "hyper 0.13.8",
  "lighthouse_version",
  "logging",
@@ -1781,6 +1783,7 @@ dependencies = [
  "beacon_chain",
  "eth2_ssz",
  "eth2_ssz_derive",
+ "hex 0.4.2",
  "proto_array",
  "slot_clock",
  "state_processing",