The wal_dir suffers from a bad option path after finishing checkpoint

### Expected behavior
If wal_dir specified, the wal_dir in option file generated by checkpoint should point to checkpoint dir.
### Actual behavior
The wal_dir in option file generated by checkpoint points to the old wal path.
### Steps to reproduce the behavior
Take a look at the following example：
```
#include
#include
#include
#include

#include "rocksdb/db.h"
#include "rocksdb/slice.h"
#include "rocksdb/options.h"
#include "rocksdb/utilities/checkpoint.h"

using namespace rocksdb;

std::string kDBPath = "./db";

int main() {
Options options;
options.create_if_missing = true;
DB* db;
Status s = DB::Open(options, kDBPath, &db);
assert(s.ok());

ColumnFamilyHandle* cf;
s = db->CreateColumnFamily(ColumnFamilyOptions(), "new_cf", &cf);
assert(s.ok());

delete cf;
delete db;

std::vector column_families;
column_families.push_back(ColumnFamilyDescriptor(
kDefaultColumnFamilyName, ColumnFamilyOptions()));
column_families.push_back(ColumnFamilyDescriptor(
"new_cf", ColumnFamilyOptions()));
std::vector handles;
DBOptions db_options;
db_options.wal_dir = kDBPath + "_wal";
s = DB::Open(db_options, kDBPath, column_families, &handles, &db);
assert(s.ok());

s = db->Put(WriteOptions(), handles[1], Slice("key"), Slice("value"));
assert(s.ok());
std::string value;
s = db->Get(ReadOptions(), handles[1], Slice("key"), &value);
assert(s.ok());

// atomic write
WriteBatch batch;
batch.Put(handles[0], Slice("key2"), Slice("value2"));
batch.Put(handles[1], Slice("key3"), Slice("value3"));
batch.Delete(handles[0], Slice("key"));
s = db->Write(WriteOptions(), &batch);
assert(s.ok());

Checkpoint* checkpoint = nullptr;
s = Checkpoint::Create(db, &checkpoint);
assert(s.ok());

std::unique_ptr checkpoint_guard(checkpoint);
std::string snapshot_name = kDBPath + "_ck";

s = checkpoint->CreateCheckpoint(snapshot_name, std::numeric_limits::max());
assert(s.ok());

for (auto handle : handles) {
delete handle;
}
delete db;

return 0;
}
```

The directory structure is shown below after the example executed.
```
├── db
│   ├── 000005.log
│   ├── CURRENT
│   ├── IDENTITY
│   ├── LOCK
│   ├── LOG
│   ├── LOG.old.1628671900840290
│   ├── MANIFEST-000010
│   ├── OPTIONS-000009
│   └── OPTIONS-000013
├── db_ck
│   ├── 000011.log
│   ├── CURRENT
│   ├── MANIFEST-000010
│   └── OPTIONS-000013
├── db_wal
│   └── 000011.log
```
The wal_dir in db_ck/OPTIONS-000013 points to ./db_wal