Balances in wallet's balance fields not updated in database
Currently, as wallet sync can take a long time, it is delegated to a dramatiq task.
It calls balance method and saves result in balance field. (it is done to make daemon restore and load wallet, afterwards all calls are fast)
But after that, balance field never updates.
But `/crud/stats` endpoint calls balance on each wallet, displaying it in stats.
We should make `balance` field a field computed on-the-fly and not stored in database. 
Also an in-memory (or redis) cache for 5 minutes could be enabled for balance field and `/crud/stats` endpoint.
The task will still call balance or help method to sync the wallet