[Enhancement] Optimize NodeSelector#seqChooseBackendOrComputeId()

[lmagic233]

Why I'm doing:

Shared-data clusters do not necessarily (or should not) have BE nodes, thus invoking NodeSelector#seqChooseBackendOrComputeId() method will produce failed to find any backend logs, which are somewhat confusing.

What I'm doing:

Refactor that method to check for CN nodes under shared-data mode first, then check for BE under both modes.

What type of PR is this:

• [ ] BugFix
• [ ] Feature
• [x] Enhancement
• [ ] Refactor
• [ ] UT
• [ ] Doc
• [ ] Tool

Does this PR entail a change in behavior?

• [ ] Yes, this PR will result in a change in behavior.
• [x] No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• [ ] Interface/UI changes: syntax, type conversion, expression evaluation, display information
• [ ] Parameter changes: default values, similar parameters but with different default values
• [ ] Policy changes: use new policy to replace old one, functionality automatically enabled
• [ ] Feature removed
• [ ] Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• [ ] I have added test cases for my bug fix or my new feature
• [ ] This pr needs user documentation (for new or modified features or behaviors)
  • [ ] I have added documentation for my new feature or new function
• [ ] This is a backport pr

Bugfix cherry-pick branch check:

• [x] I have checked the version labels which the pr will be auto-backported to the target branch
  • [x] 3.3
  • [ ] 3.2
  • [ ] 3.1
  • [ ] 3.0
  • [ ] 2.5