feat(server): implement TwoHalves buffer

[numinnex]

This PR introduces an TwoHalves buffer, that uses Vec<u8> instead of Bytes as it's underlying storage.

TwoHalves splits the underlying storage into two parts (mutable and non-mutable one). The non-mutable part uses thread-safe reference counting for cheap sharing (Bytes like), where the mutable one is copied over on each clone.

In the initial state both the mutable and non-mutable parts point to the same allocation using exactly the same control block, the diverge happens on first clone when the mutable part gets copied over and re-allocated entirely.

[codecov]

Codecov Report

❌ Patch coverage is 92.55952% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.86%. Comparing base (f0e8578) to head (69220df).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
core/iobuf/src/lib.rs	92.55%	25 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2944      +/-   ##
============================================
+ Coverage     71.81%   71.86%   +0.05%     
  Complexity      930      930              
============================================
  Files          1116     1117       +1     
  Lines         92616    92952     +336     
  Branches      70139    70484     +345     
============================================
+ Hits          66512    66803     +291     
- Misses        23543    23579      +36     
- Partials       2561     2570       +9     

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csharp	67.43% <ø> (-0.21%)	⬇️
go	36.38% <ø> (ø)
java	62.08% <ø> (ø)
node	91.37% <ø> (-0.15%)	⬇️
python	81.43% <ø> (ø)
rust	72.60% <92.55%> (+0.09%)	⬆️

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Files with missing lines	Coverage Δ
core/iobuf/src/lib.rs	92.55% <92.55%> (ø)

... and 13 files with indirect coverage changes

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[hubcio]

What about using AlignedVec as underlying buffer? this way we should be able to seamlessly switch to direct-io later on. See #2921

[numinnex]

What about using AlignedVec as underlying buffer? this way we should be able to seamlessly switch to direct-io later on. See #2921

I am in the process of looking at #2921, the next logical step is to introduce alignment both for the inner allocation, aswell as for the split_at boundry.

[numinnex]

I've added at the end of the file an comment with an proposed design for the TwoHalves aswell as Frozen buffer.

I am not sure about the design and the trade-off that is offer, would like to hear your opinions @krishvishal @hubcio

[numinnex]

I've decided against using the proposed new design, as it would make using AlignedVec extremely inefficient, requiring large amount of padding between ControlBlock and the data pointer.