utilities/backupable : Fix coverity issues
utilities/backupable : Fix coverity issues

Summary:
1. Class BackupMeta
```
52      : timestamp_(0), size_(0), meta_filename_(meta_filename),

CID 1168103 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
2. uninit_member: Non-static class member sequence_number_ is not initialized in this constructor nor in any functions that it calls.
153        file_infos_(file_infos), env_(env) {}
```
2. class BackupEngineImpl
```
513  }
        7. uninit_member: Non-static class member latest_backup_id_ is not initialized in this constructor nor in any functions that it calls.

CID 1322803 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
9. uninit_member: Non-static class member latest_valid_backup_id_ is not initialized in this constructor nor in any functions that it calls.
514}
```
3. struct BackupAfterCopyOrCreateWorkItem
```
368  struct BackupAfterCopyOrCreateWorkItem {
369    std::future<CopyOrCreateResult> result;
        1. member_decl: Class member declaration for shared.
370    bool shared;
        3. member_decl: Class member declaration for needed_to_copy.
371    bool needed_to_copy;
        5. member_decl: Class member declaration for backup_env.
372    Env* backup_env;
373    std::string dst_path_tmp;
374    std::string dst_path;
375    std::string dst_relative;
        2. uninit_member: Non-static class member shared is not initialized in this constructor nor in any functions that it calls.
        4. uninit_member: Non-static class member needed_to_copy is not initialized in this constructor nor in any functions that it calls.

CID 1396122 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
6. uninit_member: Non-static class member backup_env is not initialized in this constructor nor in any functions that it calls.
376    BackupAfterCopyOrCreateWorkItem() {}
```
4. struct CopyOrCreateWorkItem
```
318  struct CopyOrCreateWorkItem {
319    std::string src_path;
320    std::string dst_path;
321    std::string contents;
        1. member_decl: Class member declaration for src_env.
322    Env* src_env;
        3. member_decl: Class member declaration for dst_env.
323    Env* dst_env;
        5. member_decl: Class member declaration for sync.
324    bool sync;
        7. member_decl: Class member declaration for rate_limiter.
325    RateLimiter* rate_limiter;
        9. member_decl: Class member declaration for size_limit.
326    uint64_t size_limit;
327    std::promise<CopyOrCreateResult> result;
328    std::function<void()> progress_callback;
329
        2. uninit_member: Non-static class member src_env is not initialized in this constructor nor in any functions that it calls.
        4. uninit_member: Non-static class member dst_env is not initialized in this constructor nor in any functions that it calls.
        6. uninit_member: Non-static class member sync is not initialized in this constructor nor in any functions that it calls.
        8. uninit_member: Non-static class member rate_limiter is not initialized in this constructor nor in any functions that it calls.

CID 1396123 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
10. uninit_member: Non-static class member size_limit is not initialized in this constructor nor in any functions that it calls.
330    CopyOrCreateWorkItem() {}
```
5. struct RestoreAfterCopyOrCreateWorkItem
```
struct RestoreAfterCopyOrCreateWorkItem {
410    std::future<CopyOrCreateResult> result;
        1. member_decl: Class member declaration for checksum_value.
411    uint32_t checksum_value;

CID 1396153 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
2. uninit_member: Non-static class member checksum_value is not initialized in this constructor nor in any functions that it calls.
412    RestoreAfterCopyOrCreateWorkItem() {}
```
Closes https://github.com/facebook/rocksdb/pull/3131

Differential Revision: D6428556

Pulled By: sagar0

fbshipit-source-id: a86675444543eff028e3cae6942197a143a112c4

SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB