Summary:
RocksDB allows user-specified custom comparators which may not be known to `ldb`,
a built-in tool for checking/mutating the database. Therefore, column family comparator
names mismatch encountered during manifest dump should not prevent the dumping from
proceeding.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8216

Test Plan:
```
make check
```

Also manually do the following
```
KEEP_DB=1 ./db_with_timestamp_basic_test
./ldb --db=<db> manifest_dump --verbose
```
The ldb should succeed and print something like:
```
...
--------------- Column family "default"  (ID 0) --------------
log number: 6
comparator: <TestComparator>, but the comparator object is not available.
...
```

Reviewed By: ltamasi

Differential Revision: D27927581

Pulled By: riversand963

fbshipit-source-id: f610b2c842187d17f575362070209ee6b74ec6d4