logutil: fix data race when Pause is called concurrently

[alexey-igrychev]

Problem

logutil.Pause mutates the shared logger's output on every call:

func Pause(l *logrus.Logger) func() {
	l.Formatter.Format(logrus.NewEntry(l))
	bw := newBufferedWriter(l.Out)
	l.SetOutput(bw)          // write to l.Out
	return func() { bw.resume() }
}

Printer.updateDisplay calls Pause(logrus.StandardLogger()) and holds it
(via defer resumeLogs()) for the printer's entire active period. When more
than one Printer runs at the same time in a single process — e.g. a tool
that drives several concurrent build operations — each one reads l.Out
and calls l.SetOutput on the same process-wide logrus.StandardLogger()
without synchronization:

WARNING: DATA RACE
Read at ... by goroutine A:
  logutil.Pause()            pause.go  (l.Out)
Previous write by goroutine B:
  logrus.(*Logger).SetOutput()
  logutil.Pause()            pause.go  (l.SetOutput)

Fix

Install a single refcounting writer per logger instead of swapping the
logger's output on every Pause:

• The first Pause for a logger wraps l.Out once (formatter terminal-init
  still happens while l.Out is the real terminal) and stores the wrapper in
  a small guarded map.
• Each Pause increments a counter; each resume decrements it. Output is
  buffered while the count is > 0 and flushed, in order, when it returns to 0.
• Pause never blocks on another active pause, so concurrent printers keep
  running in parallel — only the interleaving of logrus output with the
  progress display is serialized, not the work itself.

Test

util/logutil/pause_test.go (new) covers concurrent pause/resume, concurrent
first-time initialization (the raced path), overlapping-output buffering, and
resume idempotency. go test -race ./util/logutil fails on master and passes
with this change.