logrotate: fix log rotation

[elhimov]

Watchdog launches tarantool in a way that produce log to stderr and thus tarantool itself does nothing to handle log rotation, it is handled entirely at watchdog side.
Prior to this patch os.File was used in Cmd.Stdout, when watchdog launches tarantool binary and according to golang documentation (see Exec.Cmd.Stdout) in this special case output from the child process is connected directly to that file. It means that even if it is closed at watchdog side, its child process (tarantool) keeps using the original one, i.e. it proceeds with logging to the original file even after renaming it.
Now Logger is used as Cmd.Stdout, and according to the mentioned documentation separate goroutine reads output from the child process over a pipe and delivers that data to the Logger. It allows proper handling of log rotation at watchdog side.

@TarantoolBot document
Title: tt logrotate properly release descriptor of the old log file.

I didn't forget about:

• Well-written commit messages (see documentation how to write a commit message)
• Don't forget about TarantoolBot in a commit message (see example)
• Tests (see documentation for a testing package)
• Changelog (see documentation for changelog format)

Related issues:

Closes #1174

[oleg-jukovec]

It is more canonical formatting.

Suggested change

	defer logger.mu.Unlock()
	return logger.Logger.Writer().Write(p)
	defer logger.mu.Unlock()
	return logger.Logger.Writer().Write(p)

[elhimov]

Corrected.