Refactor idea: the `fetch` call chain is too complex

[dshemetov]

Reading through fetch, the callflow is:

• fetch
• cache_epidata_call
• either fetch_tbl or fetch_classic
  • fetch_tbl checks if the method only supports classic and errors out, but then confusingly proceeds to call fetch_classic anyway
  • fetch_classic makes a request

My preference would be:

• absorb the fetch_tbl and fetch_classic logic into a unified imperative fetch
• pull the actual fetching part out of cache_epidata_call and only do the is_cacheable checking and storing there after the fetch has gone through