[issue#1498] db.option.wal_dir set

[Brokenice0415]

fix #1498

What is the purpose of the change

Set db wal_dir prefix to make wal path valid when creating checkpoint

[ghost]

Status Storage::Open(const StorageOptions& storage_options, const std::string& db_path)

1. 这个函数在传递的时候已经传递了strings/hashes/sets/lists/zsets
2. 这个问题应该是函数传递path的时候最后缺少一个/

`
Status Storage::Open(const StorageOptions& storage_options, const std::string& db_path) {
mkpath(db_path.c_str(), 0755);

strings_db_ = new RedisStrings(this, kStrings);
Status s = strings_db_->Open(storage_options, AppendSubDirectory(db_path, "strings/"));
if (!s.ok()) {
LOG(FATAL) << "open kv db failed, " << s.ToString();
}

hashes_db_ = new RedisHashes(this, kHashes);
s = hashes_db_->Open(storage_options, AppendSubDirectory(db_path, "hashes/"));
if (!s.ok()) {
LOG(FATAL) << "open hashes db failed, " << s.ToString();
}

sets_db_ = new RedisSets(this, kSets);
s = sets_db_->Open(storage_options, AppendSubDirectory(db_path, "sets/"));
if (!s.ok()) {
LOG(FATAL) << "open set db failed, " << s.ToString();
}

lists_db_ = new RedisLists(this, kLists);
s = lists_db_->Open(storage_options, AppendSubDirectory(db_path, "lists/"));
if (!s.ok()) {
LOG(FATAL) << "open list db failed, " << s.ToString();
}

zsets_db_ = new RedisZSets(this, kZSets);
s = zsets_db_->Open(storage_options, AppendSubDirectory(db_path, "zsets/"));
if (!s.ok()) {
LOG(FATAL) << "open zset db failed, " << s.ToString();
}
is_opened_.store(true);
return Status::OK();
}`

[Brokenice0415]

1. 这个函数在传递的时候已经传递了strings/hashes/sets/lists/zsets

感谢提醒，没有注意到

2. 这个问题应该是函数传递path的时候最后缺少一个/

不是的，还是因为wal_dir的漏设置和rocksdb的相关处理导致的丢失

New

取消在db打开时的wal_dir赋值
增加对wal_dir的判断逻辑：当wal_dir为空时，也就是实际等同db path时，使用db path的值替换它