chore: tidy btree related code

[zz-jason]

What's changed and how does it work?

tidy btree node and worker thread code, improve code readibility

[codecov]

Codecov Report

Attention: Patch coverage is 60.90909% with 86 lines in your changes missing coverage. Please review.

Project coverage is 57.32%. Comparing base (553012d) to head (16da3b0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/btree/core/BTreeGeneric.cpp	20.58%	24 Missing and 3 partials ⚠️
src/btree/core/BTreeNode.cpp	59.37%	23 Missing and 3 partials ⚠️
...clude/leanstore/btree/core/PessimisticIterator.hpp	64.00%	0 Missing and 9 partials ⚠️
include/leanstore/btree/core/BTreeNode.hpp	85.71%	4 Missing and 3 partials ⚠️
src/btree/BasicKV.cpp	12.50%	5 Missing and 2 partials ⚠️
...nstore/btree/core/PessimisticExclusiveIterator.hpp	0.00%	1 Missing and 2 partials ⚠️
src/concurrency/HistoryStorage.cpp	57.14%	2 Missing and 1 partial ⚠️
include/leanstore/btree/core/BTreeGeneric.hpp	88.88%	0 Missing and 2 partials ⚠️
src/btree/TransactionKV.cpp	66.66%	1 Missing ⚠️
src/btree/Tuple.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
+ Coverage   56.99%   57.32%   +0.32%     
==========================================
  Files          97       97              
  Lines        7044     7037       -7     
  Branches     3382     3165     -217     
==========================================
+ Hits         4015     4034      +19     
+ Misses       2657     2599      -58     
- Partials      372      404      +32     

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