Update asset stats to handle eviction

[tamirms]

The /assets response includes the following fields (code):

• number of contracts which have active balances holding the asset
• total amount of the asset held by all contracts in active balances
• number of contracts which have expired but not yet evicted balances holding the asset
• total amount of the asset held by all contracts in expired but not yet evicted balances

We can maintain active balances because LEDGER_ENTRY_RESTORE is functionally equivalent to LEDGER_ENTRY_CREATED changes.

However, we might need to remove or rename the fields which reflect stats for expired but not yet evicted contract asset balances. The fields are currently named archived_contracts_amount and num_archived_contracts which will be misleading because these counts don't include contract asset balances which have been evicted.