Fetch ERC20-specific transactions more efficiently or less frequently

[`SingleChainTransactionDataCoordinator.autoDetectERC20Transactions()`](https://github.com/AlphaWallet/alpha-wallet-ios/blob/master/AlphaWallet/Transactions/Coordinators/SingleChainTransactionDataCoordinator.swift#L71) is being called every 15 seconds for each enabled network.

It looks like it will attempt to fetch all the last X transactions even if there are newer transactions since the last fetch. Let's either:

1. Fetch only the newer ones since the last fetch (if the API supports it) or:
2. Fetch less frequently