Use PHP temp/memory stream instead of concat data in pure RAM

[DeepDiver1975]

    @jnweiger slightly related: 

files_antivirus/lib/Scanner/ICAPScanner.php

Lines 32 to 34 in b01b869

	public function onAsyncData($data) {
	$this->data .= $data;
	}

Such implementation will finally put the entire file content into RAM at the OC side. Which is basically a suboptimal approach.
onAsyncData method is supposed to send the data to the external scanner immediately instead of storing it.

Originally posted by @VicDeo in #437 (comment)