Decouple BigchainDB instances from each other

[sohkai]

Right now, the initialization of accounts and assets (see init_accounts and 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.