Tidy some TODOs (#1721)

## Issue Addressed

- Resolves #1705

## Proposed Changes

Cleans up some of my TODOs in the code base.

- Adds link to issue in this repo for BLST `unsafe` block.
- Confirms that the `nextaccount` field *is* required on an EIP-2386 wallet.
    - Reference: https://github.com/mcdee/EIPs/blob/master/EIPS/eip-2386.md#json-schema
- Removes TODO about Zeroize on bip39 that was resolved in #1701 
- Removes a TODO about an early randao reveal since we use the slot clock to generate the reveal: https://github.com/sigp/lighthouse/blob/c4bd9c86e6bf23dd66bde150ef85e08caa5e4826/validator_client/src/block_service.rs#L212-L220

## Additional Info

NA

validator_client/src/validator_store.rs

@@ -163,7 +163,6 @@ impl<T: SlotClock + 'static, E: EthSpec> ValidatorStore<T, E> {
     }
 
     pub fn randao_reveal(&self, validator_pubkey: &PublicKey, epoch: Epoch) -> Option<Signature> {
-        // TODO: check this against the slot clock to make sure it's not an early reveal?
         self.validators
             .read()
             .voting_keypair(validator_pubkey)