Go log: custom writer for '<timestamp>: msg' line format

Go's standard log package always ends its timestamp header with a space, and Lmsgprefix with prefix ": " gives "ts : msg", not "ts: msg". To get an exact "<timestamp>: <msg>" format, call log.SetFlags(0) and wrap the output in a tiny io.Writer that prepends time.Now().Format(...) + ": ". Each log.Printf call is one Write, so it's one line per call, and it works with io.MultiWriter for stdout plus a file. Related gotcha: if you count bytes on accepted net/rpc connections to measure protocol bandwidth, any admin RPCs on the same listener (e.g. distributed grep over logs) get counted too, so keep them off during benchmarks or use a separate listener.