ERC20 balance lookup is N× `eth_call` per address

[pragmaxim]

Problem

When building address token balances, Blockbook calls eth_call once per ERC20 contract. For accounts with many tokens this becomes a major latency and throughput bottleneck

Impact

• Slower address API responses for token‑rich accounts.
• Increased RPC load and rate‑limit pressure.
• Lower indexing/query throughput.

Proposed solution

• ERC20 balances are fetched in batch where supported.
• Fallback to per‑contract calls when batching is not available or a batch item fails.