mailbox/listing: Make server response for large mailbox listing faster when using threaded view

Symptom
=======
When using roundcube with mailboxes with over 60k messages, list
view was way faster than viewing in threaded view.

Mailbox index view timing:    ~360 ms
Mailbox threaded view timing: ~800 ms

Resolution
==========
Use native PHP array manipulation functions instead of rolling custom
string data reversal implementation using strpos() and substr() in a
'while' loop.

This optimization is already present in index view handler, but was missing
from threaded view.

Results after optimization
==========================
Both average out around ~360 ms response time.