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.