[instant] Add default gasPrice of 6 gwei to transactions

[BMillman19]

Description

• Default to 6 gwei for transactions

Testing instructions

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• Prefix PR title with [WIP] if necessary.
• Prefix PR title with bracketed package name(s) corresponding to the changed package(s). For example: [sol-cov] Fixed bug.
• Add tests to cover changes as needed.
• Update documentation as needed.
• Add new entries to the relevant CHANGELOG.jsons.

[coveralls]

Coverage increased (+0.009%) to 84.721% when pulling aeec8f4 on feature/instant/default-gas-price into fdf9e86 on development.