Summary:
currently `ImmutableDBOptions::Dump` use default value for `concurrent_prepare` and `manual_wal_flush`, because DBOptions ctor does not init those member variables.
so in LOG file, it will be
```
Options.concurrent_prepare: 0
Options.manual_wal_flush: 0
```
Closes https://github.com/facebook/rocksdb/pull/2864
Differential Revision: D5816240
Pulled By: ajkr
fbshipit-source-id: 82335e8bcae3dceedc6a99224e7998de5fad1e50
dcdf073654