`get_delegate_identities` returns results very slowly

[basfroman]

Steps for reproduce:

• run the command btcli st remove --network finney
• type netuid
• type wallet name
• type wallet hotkey
  And see how long takes Retrieving subnet data & identities from finney...

[thewhaleking]

Looks like the speed issue comes from SubtensorInterface.fetch_coldkey_hotkey_identities