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.