The old thread code caused time outs with a message set of 15000 messages so

The old thread code caused time outs with a message set of 15000 messages so
I started debugging, found out that preg_split was extremely slow and
started to avoid that call. I ended up in removing a lot of overhead and
working thread code.

Next step is adapting the indent array and add more status info per node in
order to generate nicer trees.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@10726 7612ce4b-ef26-0410-bec9-ea0150e637f0