Fix the problem with stdout buffering

[LeonidVas]

When a new instance of tarantool is started in instance.go, by default the log will be written with block buffering. The stdbuf utility was used to change this behavior, but it is not installed on macOS by default. Needs to investigate the problem and implement a universal solution.

// Start starts the Instance with the specified parameters.
func (inst *Instance) Start() error {
  // By default (when using "glibc") "stdout" is line buffered when connected
  // to a TTY and block buffered (one page 4KB) when connected to a pipe / file.
  // This is not how we want to log work, so set "stdout" to line buffered mode
  // by using "stdbuf" utility. "strderr" is set to no-buffering by default.
  //
  // Several useful links:
  // https://www.pixelbeat.org/programming/stdio_buffering/
  // https://man7.org/linux/man-pages/man3/setbuf.3.html
  // https://github.com/coreutils/coreutils/blob/master/src/stdbuf.c
  inst.Cmd = exec.Command("stdbuf", "-o", "L",
    inst.tarantoolPath, "-e", instanceLauncher)
  inst.Cmd.Stdout = inst.logger.Writer()
  inst.Cmd.Stderr = inst.logger.Writer()
  inst.Cmd.Env = append(os.Environ(), "TT_CLI_INSTANCE="+inst.appPath)
  inst.Cmd.Env = append(inst.Cmd.Env,
    "TT_CLI_CONSOLE_SOCKET="+inst.consoleSocket)

  // Imitate the "tarantoolctl".
  inst.Cmd.Env = append(inst.Cmd.Env, "TARANTOOLCTL=true")
  // Set the sign that the program is running under "tt".
  inst.Cmd.Env = append(inst.Cmd.Env, "TT_CLI=true")

  // Start an Instance.
  if err := inst.Cmd.Start(); err != nil {
    return err
  }
  inst.done = false

  return nil
}

Note that the original problem may have been misunderstood by the author.

[Totktonada]

Everything is okay with logs: they're written using pure write() (to stderr by default). Things are worse with print(), which is fwrite() / putchar() under the hood and is written to stdout if tarantool is not daemonized (using box.cfg({background = true})).

Please, clarify the issue.