lockfree session

the main problem is that pimcore uses session_start and not session_write_close. session_start by defaut locks the session file, so you are not able to start 2 times the same session. php just waits for the session release - but pimcore never release the session, you have to wait until the request is finised. especially in the ajax driven backend this is very slow.
for example i did some benchmarks (open some folders, get some thumbs) and session_start() blocks for 2 - 30 seconds.

using this merge request pimcore closes the session immediately. on my local machine the backend is much more fluent using this merge request.

please review this patch carefully and play around with this issue.