Optimize vidMap locking for better concurrency

Improved locking strategy based on the understanding that:
- vidMapLock protects the vidMap pointer from concurrent swaps
- vidMap has internal locks that protect its data structures

Changes:
1. Read operations: Grab pointer with RLock, release immediately, then operate
   - Reduces lock hold time
   - Allows resetVidMap to proceed sooner
   - Methods: GetLocations, GetLocationsClone, GetVidLocations,
     LookupVolumeServerUrl, GetDataCenter

2. Write operations: Changed from Lock() to RLock()
   - RLock prevents pointer swap during operation
   - Allows concurrent readers and other writers (serialized by vidMap's lock)
   - Methods: addLocation, deleteLocation, addEcLocation, deleteEcLocation

Benefits:
- Significantly reduced lock contention
- Better concurrent performance under load
- Still prevents all race conditions

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

0 files changed, 0 insertions, 0 deletions