The quest cache fields (lock, data map, expiry map) were spread across
Server with manual lock management. The old read path also missed the
RLock entirely, creating a data race. Encapsulating in a dedicated type
fixes the race and reduces Server's field count by 2.
The config package used `package _config` with a leading underscore,
which is unconventional in Go. Rename to `package config` (matching the
directory name) and use `cfg` as the standard import alias across all
93 importing files.
Resolve conflict in handlers_stage.go: keep lock-free packet
building pattern (copy session list, release lock, then build)
over upstream's in-lock QueueSendMHF approach.
Fix test compilation: remove objectIDs field references after
upstream removed it from Server struct.
Resync vendor directory with updated go.mod dependencies.
