Decouple BigchainDB instances from each other

Right now, the initialization of accounts and assets (see [init_accounts](./init_accounts.py) and [init_assets](./init_assets)) use a single BigchainDB configuration to initialize both instances of BigchainDB. This forces the two instances to run with the same configuration, and the need to enforce their database by using the `BIGCHAINDB_DATABASE_NAME` environment variable. This behaviour is **very** confusing and unexpected.

Ideally, the initialization of accounts and assets would be done only for a single BigchainDB instance, freeing the instances to use their own configurations. This also makes it easier in the future to add more ledger instances, and manage them.