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.
There are no changes visible from the user side (at least for those
users who doesn't put Prometheus's or pprof's port in quotes), just
internal refactoring. From now and on, BasicService configuration is
used by RPC server config, TLS for RPC server, pprof and Prometheus.
