support customize db opener in StartCmd

[yihuang]

Description

We use this to tune rocksdb options: crypto-org-chain/cronos#813

For example, this is how we tune the rocksdb options:

func openDB(rootDir string, backendType dbm.BackendType) (dbm.DB, error) {
	dataDir := filepath.Join(rootDir, "data")
	if backendType == dbm.RocksDBBackend {
		// customize rocksdb options
		db, err := grocksdb.OpenDb(newRocksdbOptions(), filepath.Join(dataDir, "application.db"))
		if err != nil {
			return nil, err
		}
		ro := grocksdb.NewDefaultReadOptions()
		wo := grocksdb.NewDefaultWriteOptions()
		woSync := grocksdb.NewDefaultWriteOptions()
		woSync.SetSync(true)
		return dbm.NewRocksDBWithRawDB(db, ro, wo, woSync), nil
	} else {
		return dbm.NewDB("application", backendType, dataDir)
	}
}

The NewRocksDBWithRawDB api come from our fork of tm-db: https://github.com/crypto-org-chain/tm-db/tree/release/v0.6.x

---

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---

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[fedekunze]

LGTM, minor comments.

It would be great to provide an example in the PR description

[yihuang]

LGTM, minor comments.

It would be great to provide an example in the PR description

done

[fedekunze]

@Mergifyio backport release/v0.21.x

[mergify]

backport release/v0.21.x

✅ Backports have been created

• #1622 support customize db opener in StartCmd (backport #1615) has been created for branch release/v0.21.x