fetcher: ignore fetch requests for empty hash or goldenAtx

[fasmat]

Description

At the moment the Handler in the activation package has to check if the ATX it is trying to fetch is types.EmptyATX or the golden ATX before calling GetAtxIds on the fetcher. Instead the fetcher itself should not try to fetch ATXs with those IDs but instead immediately return if instructed to fetch one of those two ATXs.

Affected code

• fetch.Fetch
• callers of fetch.Fetch::GetAtxs; this includes
  • activation.Handler
  • proposals.Handler
  • syncer.DataFetch
  • syncer.Syncer
  • possibly others

This issue appears in commit hash: 265fb39

[fasmat]

cc @countvonzero