feat: introduce valid check into iterator

[zz-jason]

What's changed and how does it work?

Ref to: #97

• add Valid() to the iterator interface

[codecov]

Codecov Report

Attention: Patch coverage is 69.51220% with 50 lines in your changes missing coverage. Please review.

Project coverage is 61.71%. Comparing base (2cc6019) to head (71e31c7).

Files	Patch %	Lines
...clude/leanstore/btree/core/PessimisticIterator.hpp	74.54%	12 Missing and 2 partials ⚠️
src/btree/TransactionKV.cpp	48.14%	9 Missing and 5 partials ⚠️
src/btree/BasicKV.cpp	0.00%	9 Missing and 1 partial ⚠️
src/leanstore-c.cpp	57.14%	6 Missing ⚠️
src/concurrency/HistoryStorage.cpp	25.00%	0 Missing and 3 partials ⚠️
...nstore/btree/core/PessimisticExclusiveIterator.hpp	33.33%	2 Missing ⚠️
tests/btree/BasicKvIteratorTest.cpp	98.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   59.89%   61.71%   +1.81%     
==========================================
  Files         118      119       +1     
  Lines        8756     8817      +61     
  Branches     3294     3469     +175     
==========================================
+ Hits         5244     5441     +197     
+ Misses       3129     3004     -125     
+ Partials      383      372      -11     

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