feat(iavl): add Pin and UnsafeBytes design for managing mmaps

[aaronc]

Description

Whenever we memory map a file we obtain a virtual memory address which becomes invalid when we unmap the file. Mapping and unmapping happen continously throughout the lifecycle of IAVLX changesets and if we reference an unmapped memory address there will be a system fault.

In order to allow for concurrent access to memory mapped files in IAVLX (a critical optimization), we use a reference counting system to make sure that the cleanup thread doesn't unmap files when the mapped memory is still in use.

In the original IAVLX implementation, this reference counting happened internally within the Changeset type in every single getter method. This was pretty messy and verbose and I wouldn't want to subject anyone to reviewing that code the way it is. In addition, it was sub-optimal performance wise because we were forced to copy any transient memory buffers which increases allocation which increases pressure on the garbage collector.

This redesign moves the responsibility for pinning and unpinning mapped memory out to the edges. While this does mean that there are a bunch of defer pin.Unpin calls whenever we access a node, the pattern itself is simpler and more like opening and closing a file. Essentially, whenever you're acess a Node you should expect that you have file like access to it and must effectively "close" it, which is sort of the case if it is mapped to a file. At the edges, however, this allows us to directly access memory mapped buffers via the UnsafeBytes abstraction which should make it clear that if you want to retain a non-transient reference to this "memory" you must copy it. This means that in query, mutation and hashing code we can now do key comparison and hash accumulation without extra allocation.

In memiavl, they had Hash and SafeHash methods on the Node interface itself to account for this behavior, but the meaning wasn't very clear or obvious. I think assuming these bytes are unsafe to retain is a better starting point. Thus the UnsafeBytes wrapper is used for returning bytes from Key, Value and Hash methods because these []byte arrays may reference mmaps.

[codecov]

Codecov Report

❌ Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.22%. Comparing base (f2d4a98) to head (02f25c9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
iavl/internal/mem_node.go	82.35%	3 Missing ⚠️
iavl/internal/node_pointer.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25657      +/-   ##
==========================================
- Coverage   70.42%   70.22%   -0.21%     
==========================================
  Files         833      835       +2     
  Lines       54395    54414      +19     
==========================================
- Hits        38310    38212      -98     
- Misses      16085    16202     +117     

Files with missing lines	Coverage Δ
iavl/internal/pin.go	100.00% <100.00%> (ø)
iavl/internal/unsafe_bytes.go	100.00% <100.00%> (ø)
iavl/internal/node_pointer.go	90.90% <66.66%> (ø)
iavl/internal/mem_node.go	94.44% <82.35%> (+0.21%)	⬆️

... and 3 files with indirect coverage changes

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[github-actions]

@aaronc your pull request is missing a changelog!