at_c: use atclient_connection_read/write in notify/monitor

[JeremyTubongbanua]

Throughout our code base, we do something like this:

static int atclient_get_self_key_validate_arguments(const atclient *atclient, const atclient_atkey *atkey,
                                                const char **value, const atclient_get_self_key_request_options *request_options) {
  int ret = 1;

  if (atclient == NULL) {
    ret = 1;
    atlogger_log(TAG, ATLOGGER_LOGGING_LEVEL_ERROR, "atclient is NULL\n");
    goto exit;
  }

  if (!atclient_is_atsign_initialized(atclient)) {
    ret = 1;
    atlogger_log(TAG, ATLOGGER_LOGGING_LEVEL_ERROR, "atclient_is_atsign_initialized is false\n");
    goto exit;
  }

  if (!atclient_is_atserver_connection_started(atclient)) {
    ret = 1;
    atlogger_log(TAG, ATLOGGER_LOGGING_LEVEL_ERROR, "atclient_is_atserver_connection_started is false\n");
    goto exit;
  }

  if (atclient->async_read) {
    atlogger_log(TAG, ATLOGGER_LOGGING_LEVEL_ERROR,
                 "atclient_get_self_key cannot be called from an async_read atclient, it will cause a race condition\n");
    return 1;
  }

  if (atkey == NULL) {
    ret = 1;
    atlogger_log(TAG, ATLOGGER_LOGGING_LEVEL_ERROR, "atkey is NULL\n");
    goto exit;
  }

  if (!atclient_atkey_is_key_initialized(atkey)) {
    ret = 1;
    atlogger_log(TAG, ATLOGGER_LOGGING_LEVEL_ERROR, "atkey.key is not initialized when it should be\n");
    goto exit;
  }

  if (!atclient_atkey_is_shared_by_initialized(atkey)) {
    ret = 1;
    atlogger_log(TAG, ATLOGGER_LOGGING_LEVEL_ERROR, "atkey.shared_by is not initialized when it should be\n");
    goto exit;
  }

  ret = 0;
exit: { return ret; }
}

We should follow the same pattern to avoid segfaults in notify/monitor