properly cancel a Paginated Results operation in order to avoid protocol...

... errors, fixes #10526

This is mostly an issue with OpenLDAP, AD worked with the old approach. But now we are also following the RFC more nicely.

How to test?
1. Have an OpenLDAP ready with more than 20 users
2. Put the test script into  your ownCloud root: https://gist.github.com/blizzz/1b4998422823f7a2d3af
3. Run the test script from command line: ```php -f testCancel.php  2> data/owncloud.log```

On master with OpenLDAP it will fail with this output:
```
Step 1: fetching 10 users succeeded
Step 2: reading attribute done
Validation: cancelling Paged Search failed! Users found:  0
```

With AD, or with  OpenLDAP and this fix, it will succeed, output like this:
```
Step 1: fetching 10 users succeeded
Step 2: reading attribute done
Validation: cancelling Paged Search succeeded :) !
```

If possible, please also test with 389 Directory Server, OpenDJ, Oracle Internet Directory, Apache DS.