Do not use the _ for new identifier in DataHandler

[DanielSiepmann]

The _ inside the uid has a special meaning, see

if (strpos($value, '_') === false) {
    $affectedTable = $tcaFieldConf['foreign_table'];
    $prependTable = false;
} else {
    $parts = explode('_', $value);
    $value = array_pop($parts);
    $affectedTable = implode('_', $parts);
    $prependTable = true;
}
$value = $this->substNEWwithIDs[$value];
// The record is new, but was also auto-versionized and has another new id:
if (isset($this->autoVersionIdMap[$affectedTable][$value])) {
    $value = $this->autoVersionIdMap[$affectedTable][$value];
}
if ($prependTable) {
    $value = $affectedTable . '_' . $value;
}
// Set a hint that this was a new child record:
$this->newRelatedIDs[$affectedTable][] = $value;
$valueArray[$key] = $value;

https://github.com/TYPO3/TYPO3.CMS/blob/10.4/typo3/sysext/core/Classes/DataHandling/DataHandler.php#L6249

We should not use the _ to follow the documented and expected behaviour.

The _ has no consequences right now, because we do not use the uids in relations yet. But this might change in the future and would result in broken behaviour.