feat: ADR-040: Implement BadgerDB backend (#9848)

## Description

Partially resolves: vulcanize#14

Implements a [BadgerDB](https://pkg.go.dev/github.com/dgraph-io/badger/v3)-based backend for the DB interface introduced by #9573 and specified by [ADR-040](https://github.com/cosmos/cosmos-sdk/blob/eb7d939f86c6cd7b4218492364cdda3f649f06b5/docs/architecture/adr-040-storage-and-smt-state-commitments.md). This uses Badger's "managed" mode for version management, and supports optimistically concurrent transactions (required one [patch](dgraph-io/badger#1716) upstream to fix handling of write conflicts).

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules) - n/a
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -151,6 +151,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * [\#8518](https://github.com/cosmos/cosmos-sdk/pull/8518) Help users of multisig wallets debug signature issues.
 * [\#9573](https://github.com/cosmos/cosmos-sdk/pull/9573) ADR 040 implementation: New DB interface
 * [\#9952](https://github.com/cosmos/cosmos-sdk/pull/9952) ADR 040: Implement in-memory DB backend
+* [\#9848](https://github.com/cosmos/cosmos-sdk/pull/9848) ADR-040: Implement BadgerDB backend
 
 
 ### Client Breaking Changes

db/README.md

@@ -37,3 +37,28 @@ This represents a self-contained and immutable view of a database's version hist
 
 The in-memory DB in the `db/memdb` package cannot be persisted to disk. It is implemented using the Google [btree](https://pkg.go.dev/github.com/google/btree) library.
   * This currently does not perform write conflict detection, so it only supports a single open write-transaction at a time. Multiple and concurrent read-transactions are supported.
+
+### BadgerDB
+
+A [BadgerDB](https://pkg.go.dev/github.com/dgraph-io/badger/v3)-based backend. Internally, this uses BadgerDB's ["managed" mode](https://pkg.go.dev/github.com/dgraph-io/badger/v3#OpenManaged) for version management.
+Note that Badger only recognizes write conflicts for rows that are read _after_ a conflicting transaction was opened. In other words, the following will raise an error:
+
+```go
+tx1, tx2 := db.Writer(), db.ReadWriter()
+key := []byte("key")
+tx2.Get(key)
+tx1.Set(key, []byte("a"))
+tx2.Set(key, []byte("b"))
+tx1.Commit()        // ok
+err := tx2.Commit() // err is non-nil
+```
+
+But this will not:
+```go
+tx1, tx2 := db.Writer(), db.ReadWriter()
+key := []byte("key")
+tx1.Set(key, []byte("a"))
+tx2.Set(key, []byte("b"))
+tx1.Commit() // ok
+tx2.Commit() // ok
+```