update estimate_gas interface everywhere

[FlorianFranzen]

What was wrong?

Documentation and mock interfaces still use the old format, while the actual implementation uses the new interface already.

Closes #181

How was it fixed?

Update documentation and mock interface by add optional block_number parameter.

Todo:

• [x] Clean up commit history
• [x] Add or update documentation related to these changes
• [x] Add entry to the release notes

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