feat: impl memtable and memtable iterator

[MrCroxx]

Danger: unsafe impl Send + Sync for Node (read_only + atomic) and IterRef (inner always Sync + Send). Check twice!!!

• Implement Memtable and MemtableIterator.
• Fix Comparator::same_key.
• Fix timestamp encoding, bug: use big end encoding for timestamp #47 .
• Fix MemtableIterator handle multi version and tombstone.
• Add unit tests for MemtableIterator.
• Add concurrent unit tests for Memtable.

Ref: #44 .
Close: #47 .

[codecov-commenter]

Codecov Report

Merging #46 (f2fb3e9) into main (97bc5c6) will increase coverage by 0.77%.
The diff coverage is 93.18%.

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   89.56%   90.33%   +0.77%     
==========================================
  Files          26       28       +2     
  Lines        2433     2555     +122     
==========================================
+ Hits         2179     2308     +129     
+ Misses        254      247       -7     

Impacted Files	Coverage Δ
storage/src/lsm_tree/iterator/mod.rs	66.66% <ø> (ø)
storage/src/lsm_tree/utils/skiplist/list.rs	72.90% <ø> (+13.54%)	⬆️
storage/src/lsm_tree/components/memtable.rs	70.58% <70.58%> (ø)
rudder/src/meta.rs	10.34% <75.00%> (-72.99%)	⬇️
storage/src/lsm_tree/iterator/memtable_iterator.rs	97.16% <97.16%> (ø)
storage/src/lsm_tree/iterator/sstable_iterator.rs	93.36% <100.00%> (ø)
storage/src/lsm_tree/utils/skiplist/arena.rs	92.85% <0.00%> (+7.14%)	⬆️

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