Fork choice specs and implementation issues: attestations

[ericsson49]

List of differences, that I found, between fork choice/attestation spec and our implementation. This doesn't necessarily mean it's a bug.

1 Attestations are delayed until referenced block arrives, but not delayed until a proper slot starts.
Spec says

# Attestations cannot be from future epochs. If they are, delay consideration until the epoch arrives

And also

# Attestations can only affect the fork choice of subsequent slots.
# Delay consideration in the fork choice until their slot is in the past.

2 Time checks are different

As written above, the spec assumes the epoch should arrive and a consequent slot should start. However, current Harmony implementation uses different checks regarding the time (see TimeFramFilter).

3 Checks are more restrictive in general

That partially seems to be the spec problem, but in general attestation assertions that are verified by Harmony implementation differ from the specification and they are more restrictive (ignoring the time issue above). E.g. a proper attestation according to fork choice spec may be rejected by Harmony. However, it's likely won't be included in the chain, because of process_attestation checks.
Still, according to the fork choice spec it should (or may) affect LMD GHOST decisions.

A similar problem is discussed here ethereum/consensus-specs#1406