6b4dd5703e made it to be a uint16 which was
somewhat important for RPC, but now it's irrelevant and the fact that it was a
string in some cases may lead to errors like these:
failed to unmarshal config YAML: yaml: unmarshal errors:
line 48: cannot unmarshal !!str `20011` into uint16
line 52: cannot unmarshal !!str `40001` into uint16
So for maximum backwards compatibility we better have string here and
eventually it'll be deleted anyway.
