[Enhancement] sort dicts directly to speed up BitmapIndexWriter.

[wuxueyang96]

Why I'm doing:

Currently, we use a std::map at BitmapIndexWriterImpl::finish to get sorted dictionaries and sorted ngram dictionaries. It will waste a lots of CPU and memory according to the test that its result is shown below:

It is obvious that using a std::unordered_map to deduplicate and using a std::vector to store all keys to get a sorted dictionaries could get the best performance.

What I'm doing:

Use std::unordered_map to deduplicate dictionaries and use a std::vector to get sorted dictionaries.

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:

• [x] 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
  • [ ] 4.0
  • [ ] 3.5
  • [ ] 3.4
  • [ ] 3.3

[mergify[bot]]

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