import data from Duniter 1.8 leveldb database

Import data from Duniter 1.8 leveldb database. See #2 (closed).

Add adapters to get data from leveldb indexes:

• blocks
• certifications
• identities
• memberships
• wallets
• ud_value

Script is now a little faster, because we don't load the entire .json files anymore, we just iterate on leveldb indexes.