LevelDB internal statistic print function added

[JonghyeokPark]

• Using GetProperty(), we can print LevelDB internal statistic
• GetProperty() Provides four metrics
  • leveldb.sstables : Describes all of sstables contents
  • leveldb.stats : Statistic about internal operations
  • leveldb.approximate-memory-usage : Approximate number of bytes of memory in use by DB
  • leveldb.num-files-at-level : Number of files at level
• In default, we disable printing leveldb.sstables

LevelDB collects useful metrics, and we can exploit these functions at the wrapper level. We can also consider how to pass parameters to the wrapper level. For example, 1) enable/disable to print index-specific internal metrics, 2) optional printing metrics.

[llersch]

Great! Thanks a lot! I'm accepting the PR.

As a future note, we plan on adding some automatic check for code style, so watch out for things like consistent indentation.