Summary:
tools/ldb_cmd.cc:
```
310 ignore_unknown_options_ = IsFlagPresent(flags, ARG_IGNORE_UNKNOWN_OPTIONS);
CID 1322798 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
5. uninit_member: Non-static class member db_ttl_ is not initialized in this constructor nor in any functions that it calls.
311}
```
Closes https://github.com/facebook/rocksdb/pull/3122
Differential Revision: D6428576
Pulled By: sagar0
fbshipit-source-id: d77f04dd201f7f1d9f59ef88a215ee7ad7b934e9
c1ed005a21