Enable set-identity while waiting for the cluster during cluster restart

[diman-io]

Problem

Currently, it’s not possible to switch the identity from dummy to real until the cluster is up.

This forces you to temporarily copy the key to the node, which is not good.

If you don’t copy it and just wait, in the general case, the cluster might never start (assuming at least 20% of the stake is waiting this way).

[pires]

IIUC we're running into this too, while trying the instructions for testnet restart.
This is what we get when running echo IDENTITY | agave-validator --ledger $LEDGER set-identity:

admin rpc error: Server returned rpc error Invalid params: Retry once validator start up is complete