Balances. Balances fetching overhaul.

[johnthecat]

We've got some technical limitation in current balances fetching mechanism.

• User can see balances only for current wallet.
• With a lot of chains balance subscriptions might become a performance bottleneck.

Current idea of refactoring balances is to add priority to subscriptions. We can use 3 priorities:

• High — accounts in selected wallet in well known chains (polkadot, kusama, etc.). This subscriptions are realtime and uses polkadotjs lib storage subscription mechanism.
• Medium — accounts is selected wallet from other chains. Balances are fetched using polling once at 1-3 minutes. Also there is transfer event subscription for better UX.
• Low — all accounts that are not in selected wallet. Balances are fetched using pooling once at 5-6 minutes.

Also model should have methods for balances force update. It can be useful for certain user flows, when user need to see actual balances.

Nice to have:

• updateAt field in db records for better distribution of balance requests. Older record - higher request priority on app start.
• Cache cleanup on wallet deletion.