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

crypto/bls/src/impls/blst.rs

@@ -66,7 +66,7 @@ pub fn verify_signature_sets<'a>(
 
         // TODO: remove this `unsafe` code-block once we get a safe option from `blst`.
         //
-        // See https://github.com/supranational/blst/issues/13
+        // https://github.com/sigp/lighthouse/issues/1720
         unsafe {
             blst::blst_scalar_from_uint64(rand_i.as_mut_ptr(), vals.as_ptr());
             rands.push(rand_i.assume_init());