[cli] disallow concurrent commands (#6695)

This commit disallows concurrent commands.

With this commit, the CLI only execute a new command after it complete
the previous command. CLI can also prompt properly after the command
execution is done.

Other fixes and enhancements:
- Fixes premature command prompt
- Add ping async command for ping in async mode: output Done
  immediately but print ping responses later on.
- Fixes networkdiagnostic get outputs multiple Done by always waiting
  for 5 seconds.

tools/otci/otci/command_handlers.py

@@ -180,6 +180,8 @@ class OtCliCommandRunner(OTCommandHandler):
     def __otcli_read_routine(self):
         while not self.__should_close.isSet():
             line = self.__otcli.readline()
+            logging.debug('%s: %r', self.__otcli, line)
+
             if line.startswith('> '):
                 line = line[2:]