Chests (and lockboxes, coffers, etc.) failed to open because CMSG_LOOT
was only sent on Classic/Turtle expansions, and only when GO type was
already cached as type 3. Fix: always send CMSG_LOOT after
CMSG_GAMEOBJ_USE (server silently ignores it for non-lootable objects).
Also broaden CMSG_GAMEOBJ_REPORT_USE to all non-mailbox WotLK GOs.
Latency meter: record pingTimestamp_ in sendPing() and compute RTT in
handlePong(); add toggleable "Show Latency Meter" checkbox in Interface
settings (saved to settings.cfg).