Wrap notification generation in a transaction

[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 of the diff:

			// Add this before the begin of the transaction
			$start = microtime(true);

			// Add this after the commit of the transaction
			$end = microtime(true);
			\OC::$server->getLogger()->error('notify: ' . ($end - $start));

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

With transaction	Without transaction
~0.11s	~0.65s

This is at least backportable. I think in the end It would be better if the notification app would provide a way to to create a notification for multiple users in one call and then do the transaction itself

[nickvergessen]

/backport to stable23

[vitormattos]

Excellent results