Improve mass notification processing

[nickvergessen]

Steps

1. Create 70 users:

i=10
while [ $i -ne 70 ]
do
        i=$(($i+1))
        echo "$i"

	USERID=$(uuidgen)
	OC_PASS="123456" occ -vvv user:add --password-from-env --display-name "Dummy #$i" -g "dummies" $USERID
	occ -vvv user:setting $USERID settings email "test$i@yourdomain.tld"

done

2. Create a conversation as admin with group "dummies"
3. Start a call
4. Measure time around the section in talk:
   https://github.com/nextcloud/spreed/blob/1aaf10e2653e764ed54a6abc9eac750753263a3c/lib/Notification/Listener.php#L234-L253

			// Add this after https://github.com/nextcloud/spreed/blob/1aaf10e2653e764ed54a6abc9eac750753263a3c/lib/Notification/Listener.php#L233
			$start = microtime(true);

			// Add this before https://github.com/nextcloud/spreed/blob/1aaf10e2653e764ed54a6abc9eac750753263a3c/lib/Notification/Listener.php#L255
			$end = microtime(true);
			\OC::$server->getLogger()->error('notify: ' . ($end - $start));

5. Measure the patch again with the beginTransaction and commit being commented out

With patch	Without patch
~0.01s	~0.5s

[nickvergessen]

/backport to stable23

[vitormattos]

Sounds good!
I liked the usage of array_chunk