cmd,eth: use gas price monitor for all transactions instead of 'eth_gasPrice' from the ethereum JSON-RPC provider

[kyriediculous]

What does this pull request do? Explain your changes. (required)
Improve transaction management with better UX

• use gas price monitor for all transactions instead of 'eth_gasPrice' from the ethereum JSON-RPC provider

Specific updates (required)

• Use GasPriceMonitor.GasPrice() instead of ethClient.SuggestGasPrice() in Backend.SuggestGasPrice(). Since Backend overwrites the eth.Transactor interface all transactions, also contract calls through the contract bindings, will use the gas price stored by the GasPriceMonitor.
• Use the GasPriceMonitor regardless of the node being in O or B mode

How did you test each of these updates (required)

• Unit tests

Does this pull request close any open issues?
Fixes #1906

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[yondonfu]

Let's add a unit test checking that SuggestGasPrice on backend returns the value of GasPrice() on GasPriceMonitor?

[yondonfu]

LGTM after squashing and fixing merge conflict.

Looks like GH actions is stuck for some reason...Let's see if it runs properly after a rebase against master...

[kyriediculous]

The windows builds seem to be failing consistently for all PRs , not sure what's wrong there.

[yondonfu]

The windows builds seem to be failing consistently for all PRs , not sure what's wrong there.

Yeah it has been failing consistently for reasons unrelated to recent code changes. We'll need to fix that separately. Let's go ahead with the merge for now.