Increase default gas limit and allow changing it

[m4ksio]

Goal of this PR

Set default gas limit to maximum possible value, and allow to tune it

Checklist

Does the PR require tests to be added or updated?

No

Does the PR require documentation to be added or updated?

Updated flag docs

Misc

• [x] PR title will be clear as part of the changelog
• [x] PR is against the correct branch
• [x] PR is labelled appropriately
• [ ] PR is linked to an issue

[m4ksio]

Includes the changes from the other PR. This PR should be independent from the other one.

Right, can't change this PR to my branch as its in other repo, will close and reopen later.