Optimize GraphQL queries by adding additional indexes on DB level

[xgreenx]

The source of this issue is #623.

Initially, the idea was to remove all complex queries from fuel-core and make it part of the indexer's responsibility. But the SDK is deeply integrated with these basic queries, that we need to support them on fuel-core. For that, we need to optimize the following queries by aggregating some information in the off-chain worker or adding new indexes to the database:

• The balance and balances queries iterate over all entries of coins and messages in the storage. The off-chain worker should index the balance of each account and aggregate this information in a user-friendly form.
• The coins_to_spend query is very complex and requires n^2 operations where n is the number of coins and messages. The algorithm itself can use additional indexes from RocksDB to solve the issue with dust coins and works fast.
• The dry_run already allows the dry running of many transactions; we just need to forbid several dry run sub-queries inside of one query.