Skip local blob server for cloud stores

[zhangzhenghao]

Summary

• skip the legacy local blob gRPC server when [blob].uri uses a cloud URI scheme
• avoid registering a nil blob server in that case
• add coverage that cloud URIs do not create a local s3: directory while local paths still create a blob server

Fixes #1283

Test Plan

• /usr/local/go/bin/go test ./master -run TestNewMasterBlobServer -count=1
• /usr/local/go/bin/go test ./master -run TestMaster -count=1
• /usr/local/go/bin/go test ./... -run '^$'
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.96%. Comparing base (054c37b) to head (6eadd1b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1285      +/-   ##
==========================================
- Coverage   74.07%   73.96%   -0.11%     
==========================================
  Files          92       92              
  Lines       17256    17258       +2     
==========================================
- Hits        12782    12765      -17     
- Misses       3155     3167      +12     
- Partials     1319     1326       +7     

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