* Use Unix sockets for gRPC between co-located services in weed server
Extends the Unix socket gRPC optimization (added for mini mode in #8856)
to `weed server`. Registers Unix socket paths for each service's gRPC
port before startup, so co-located services (master, volume, filer, S3)
communicate via Unix sockets instead of TCP loopback.
Only services actually started in this process get registered. The gRPC
port is resolved early (port + 10000 if unset) so the socket path is
known before any service dials another.
* Refactor gRPC Unix socket registration into a data-driven loop
9205140bd5