Compute Circulating Supply and expose via backend

[frol]

This is only applicable to mainnet

This issue is blocked on #478

Circulating Supply = Total Supply - Tokens locked in the Lockup contracts

Implementation Notes

Here is how to compute the value:

1. Total Supply is available though RPC block method (see header->total_supply)
2. Tokens locked in the lockup contracts needs to be queried individually:
   2.1. Query Indexer for Explorer database for all the lockup accounts: SELECT account_id FROM accounts WHERE account_id LIKE '%.lockup.near'
   2.2. Use account-lookup code to query the still locked balance
3. Sum up all the locked balances and subtract it from the Total Supply

NOTE: Make sure you pin a specific block height when you query the Total Supply and every individual contract state (through the account-lookup)

Compute the value every 2 hours on the backend and store the value in memory (if the computation takes too long, consider storing it in SQLite). Expose the precomputed value via WAMP get-circulating-supply method.

Acceptance Criteria

• The value is computed correctly (compare with the projection)
• Non-mainnet networks return undefined
• The value is re-computed every 2 hours (note: adjust the number if necessary)

[shelegdmitriy]

@telezhnaya is this issue still relevant?