Optimize locking: Reduce lock acquisitions and use helper methods

Two optimizations to further reduce lock contention and improve code consistency:

1. LookupFileIdWithFallback: Eliminated redundant lock acquisition
   - Before: Two separate locks to get vidMap and dataCenter
   - After: Single lock gets both values together
   - Benefit: 50% reduction in lock/unlock overhead for this hot path

2. KeepConnected: Use GetDataCenter() helper for consistency
   - Before: Manual lock/unlock to access DataCenter field
   - After: Use existing GetDataCenter() helper method
   - Benefit: Better encapsulation and code consistency

Impact:
- Reduced lock contention in high-traffic lookup path
- More consistent use of accessor methods throughout codebase
- Cleaner, more maintainable code

Verified with: go test -race ./weed/wdclient/... (passes)

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