Refactor sel vector interface

[andyfengHKU]

This PR ends up being a pure renaming PR.

We rename resetSelectorToUnselected to setToUnfiltered to be consistent with isUnfiltered. And resetSelectorToValuePosBuffer to setToMutable. The reasons are as follow

• reset has a very specific meaning of setting to initial state. Out interface in SelVector clearly not qualify this notion.
• resetSelectorToValuePosBuffer is quite confusing because developer do not know what ValuePosBuffer is suppose to be. We should avoid name an interface by naively describing a code block (e.g. resetSelectorToValuePosBuffer is just describing selectedPositions = selectedPositionsBuffer.get();). Instead, call it setToMutable makes more sense because by default selectedPositions points to a static array that is immutable.

I also take the chance to examine if we were using SelVector wrongly. I spot a few pieces where @ray6080 should comment as it is storage code.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.21%. Comparing base (fa528c1) to head (24c39a0).

Files	Patch %	Lines
src/storage/store/column_chunk.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3177      +/-   ##
==========================================
- Coverage   92.21%   92.21%   -0.01%     
==========================================
  Files        1171     1171              
  Lines       44127    44125       -2     
==========================================
- Hits        40693    40690       -3     
- Misses       3434     3435       +1     

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