Respect block gas limit in gasEstimate buffer

[fleupold]

Description

When looking at driver native solution submissions on Gnosis Chain, I notice that we sometimes fail due to:

Error { code: ServerError(-32010), message: "GasLimitExceeded, Gas limit: 17000000, gas limit of rejected tx: 17728674", data: None }

This stems from the fact that we multiply the gasEstimate we receive during simulation by 2 (to account for inaccuracies, changes in the execution trace, refunds, etc.)
This PR makes is so that we upper bound this buffered estimate by the current block size.

Changes

• Introduce method on infra::eth to fetch the current gas limit
• Use this value when creating the gas estimate

Thanks to @MartinquaXD great suggestion I moved the gas limit fetch into the Fetch.sol helper contract, which even allows this call to be infallible and synchronous 🙌 .

How to test

Added a driver integration test

[MartinquaXD]

Note, that the call to get the current gas limit isn't cached and can potentially lead to a lot of expensive getBlock calls on our node. I'm not sure what a good and simple caching strategy is. Any suggestions?

The most straight forward solution would probably to adjust FetchBlock.sol to also return the gas limit and access that value from the current block stream. That way we don't add any additional calls and get caching for free.

[MartinquaXD]

Just a nit on the test function.