Consider making output of `List` from `Keystore` interface streamable/chunked

The current declaration of Keystore's `List() ([]string, error)` doesn't take any arguments and returns the whole list of keys per one output. I suggest making it a line-buffered stream.

My use case comes from :
> Currently the key listing accumulates all the keys and outputs them to terminal in one write.
I think, it would be better to output line-buffered stream of keys to the terminal.
It's related because the case is the same: generating keys in batches.