fix: fix crash when enable standby and streaming

[chyezh]

issue: #38125

• connect kv at standby mode.
• make balancer initialization lazy.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 81.08%. Comparing base (7944538) to head (60addad).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/datacoord/server.go	86.36%	2 Missing and 1 partial ⚠️
internal/streamingcoord/server/server.go	50.00%	2 Missing and 1 partial ⚠️
...ternal/streamingcoord/server/service/assignment.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #38239      +/-   ##
==========================================
- Coverage   81.09%   81.08%   -0.02%     
==========================================
  Files        1372     1372              
  Lines      191565   191582      +17     
==========================================
- Hits       155352   155335      -17     
- Misses      30719    30740      +21     
- Partials     5494     5507      +13     

Components	Coverage Δ
Client	74.43% <ø> (ø)
Core	68.85% <ø> (ø)
Go	83.20% <75.00%> (-0.02%)	⬇️

Files with missing lines	Coverage Δ
internal/streamingcoord/server/builder.go	100.00% <100.00%> (ø)
internal/datacoord/server.go	72.92% <86.36%> (-0.07%)	⬇️
internal/streamingcoord/server/server.go	78.78% <50.00%> (-8.72%)	⬇️
...ternal/streamingcoord/server/service/assignment.go	78.57% <40.00%> (-21.43%)	⬇️

... and 29 files with indirect coverage changes

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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This pull-request has been approved by: chyezh

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[jaime0815]

/lgtm