Summary:
Create a tool to iterate through keys and dump values. Current options
as follows:

db_dump --start=[START_KEY] --end=[END_KEY] --max_keys=[NUM] --stats
[PATH]

START_KEY: First key to start at
END_KEY: Key to end at (not inclusive)
NUM: Maximum number of keys to dump
PATH: Path to leveldb DB

The --stats command line argument prints out the DB stats before dumping
the keys.

Test Plan:
- Tested with invalid args
- Tested with invalid path
- Used empty DB
- Used filled DB
- Tried various permutations of command line options

Reviewers: dhruba, heyongqiang

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D5643