feat: ADR for api cleanup and async commit

docs/architecture/adr-002-api-cleanup.md

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+# ADR ADR-002: API Cleanup to Support Store V2
+
+## Changelog
+
+* 2023-06-06: First draft
+
+## Status
+
+DRAFT
+
+## Abstract
+
+This ADR proposes a cleanup of the API to support [Store V2](https://github.com/cosmos/cosmos-sdk/blob/main/docs/architecture/adr-065-store-v2.md).
+
+There are some proposals for the speedup of the `Commit` by the async writes. See the [Discussion](https://github.com/cosmos/cosmos-sdk/issues/16173) for more details.
+
+There is a lot of legacy code in the SDK that is not used anymore and can be removed. See the [Discussion](https://github.com/cosmos/iavl/issues/737) for more details. 
+
+## Context
+
+The introduction of `Store V2` separates SS (State Storage) and SC (State Commitment), where `iavl` is used for SC and `versionDB` is already implemented for SS. The index of the `fast node` is not needed in `iavl` since `versionDB` allows key-value queries for a specific version.
+Additionally, `Store V2` introduces a new approach to `SaveVersion` that accepts a batch object for the atomicity of the commit and removes the usage of `dbm.Batch` in `nodeDB`.
+
+The current implementation of `iavl` suffers from performance issues due to synchronous writes during the `Commit` process. To address this, the proposed changes aim to finalize the current version in memory, parallelize hash calculations in `WorkingHash`, and introduce asynchronous writes.
+
+Moreover, the existing architecture of `iavl` lacks modularity and code organization:
+
+- The boundary between `ImmutableTree` and `MutableTree` is not clear.
+- There are many public methods in `nodeDB`, making it less structured.
+- `nodeDB` serves as both storage and cache simultaneously.
+
+## Decision
+
+To support `Store V2` and improve `Commit` performance, we propose the following changes:
+
+### Support Store V2
+
+- Refactor the `SaveVersion` function to accept a batch object.
+- Remove the index of the `fast node` from `iavl`.
+- Eliminate the usage of `dbm.Batch` in `nodeDB`.
+
+### Async Commit
+
+- Parallelize hash calculations in `WorkingHash`.
+- Finalize the current version in memory during `Commit`.
+- Perform async writes in the background.
+
+### API Cleanup
+
+- Merge `ImmutableTree` and `MutableTree` into a single `Tree` structure.
+- Make `nodeDB` methods private.
+- Separate the cache from `nodeDB` and introduce a new `nodeCache` component.
+
+The modified API will have the following structure:
+
+```go
+    Tree interface {
+        Has(key []byte) (bool, error)
+        Get(key []byte) ([]byte, error)
+        SaveVersion(batch struct{
+            keys [][]btye
+            values [][]byte
+        }) ([]byte, int64, error)
+        Version() int64
+        Hash() []byte
+        WorkingHash() []byte
+        VersionExists(version int64) bool
+        DeleteVersionsTo(version int64) error
+        GetVersioned(key []byte, version int64) ([]byte, *cmtprotocrypto.ProofOps, error)
+        GetTree(version int64) (*iavl.Tree, error)
+        SetInitialVersion(version uint64)
+        Iterator(start, end []byte, ascending bool) (types.Iterator, error)
+        LoadVersionForOverwriting(targetVersion int64) error
+    }
+```
+## Consequences
+
+We expect the proposed changes to improve the performance of Commit through async writes and parallelized hash calculations. Additionally, the codebase will become more organized and maintainable.
+
+### Backwards Compatibility
+
+While this ADR will break the external API of `iavl`, it will not affect the internal state of nodeDB. Compatibility measures and migration steps will be necessary during the `Store V2` migration to handle the breaking changes.
+
+### Positive
+
+- Atomicity of the `Commit`.
+- Improved Commit performance through async writes and parallelized hash calculations.
+- Increased flexibility and ease of modification and refactoring for iavl.
+
+### Negative
+
+- Async Commit may result in increased memory usage and increased code complexity.
+
+## References
+
+- [Store V2](https://github.com/cosmos/cosmos-sdk/blob/main/docs/architecture/adr-065-store-v2.md)
+- https://github.com/cosmos/cosmos-sdk/issues/16173
+- https://github.com/cosmos/iavl/issues/737