Don't memoize eth client TransactOpts

[ian-shim]

Why are these changes needed?

I think memoizing TransactOpts is potentially dangerous pattern as it's trivial to update the memoized TransactOpts and the caller needs to remember not to.
Since generating TransactOpts isn't too expensive, generating new TransactOpts each time can prevent such scenario.

Checks

• I've made sure the lint is passing in this PR.
• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(

[mooselumph]

lgtm