Add nonblock logsink for non-blocking log writes to stderr/stdout/file

[brandond]

Fixes:

• etcd hangs indefinitely when its stdout/stderr log sink stops being drained #22326

This PR adds a nonblocking log sink implementation under the client/pkg/logutil package, and imports it from server/embed so that log outputs such as nonblock:stdout or nonblock:/var/log/etcd.log can be specified in the server config.

Since zap outputs are parsed as URLs, the log sink accepts query parameters; four are recognized:

• timeout=(time.Duration) - timeout for log writes to the sink; defaults to 1ms.
• mode=(os.FileMode) - file mode for log files; defaults to 0666.
• warn=(bool) - when writes succeed after failing due to os.ErrDeadlineExceeded, print a message to the sink counting how many writes were dropped; defaults to false.
• must=(bool) - drop the write if SetWriteDeadline cannot be used to enforce the timeout; defaults to false.

The log sink uses syscall.Dup to duplicate stdout/stderr file descriptors to a new File so that other accesses to os.Stdout or os.Stderr do not unintentionally put it into blocking mode - see golang/go#24331.

Writes that fail are silently dropped, so this is a LOSSY log sink. That said, most administrators will probably find that potentially dropping logs is better than deadlocking.

This does NOT change the default log sinks; administrators must explicitly configure log output using the nonblock scheme to use this new sink.

[kubernetes-prow]

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[kubernetes-prow]

Hi @brandond. Thanks for your PR.

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