Adjust the default GraphQL complexity for queries that can potentially use indexation

[rafal-ch]

Basically, this issue is about resolving the following TODO in the code.

pub const DEFAULT_QUERY_COSTS: Costs = Costs {
    // TODO: The cost of the `balance` and `balances` query should depend on the
    //  `OffChainDatabase::balances_enabled` value. If additional indexation is enabled,
    //  the cost should be cheaper.
    balance_query: 40001,
    coins_to_spend: 40001,

If the "balances" or "coins to spend" indexation is available in the particular instance of the client, these costs should be lower.

Please note that the default cost is no longer "const", as it depends on the flag in offchain DB metadata (indexation_availability flag).