Refactor of gas calculation

[Agupane]

As discussed internally we would need to refactor our current gas calculation, currently, we have this approach:

After the discussion with Richard we decided to go this way:

The logic of estimating the gas should be independent of the current threshold value

After the refactor we should have something like:

1) estimateGasForTxCreation: 
When tx is created -> safe.requiredTxGas

2) estimateGasForTxExecution: 
When tx is executed -> web3.estimateGas

3) estimateGasForApproval: 
- For on chain approval -> web3.estimateGas 
- For off chain approval -> no need to estimate anything
- Note: as we don't know if an approval will be off or on chain we will assume it is on chain

[mmv08]

Could you please add a little bit more info? Also, maybe link the discussion

I find it confusing that the same action, for example, "tx ready for execution" has multiple Xes for estimation methods (which one is going to be used at the end?)

Also, the table for the current implementation doesn't seem correct for the off-chain tx approval, I couldn't find a place where it calculates the gas in such a case

[nicosampler]

@mikheevm in step 2, Richard suggested first call to web3.estimateGas, then with the returned value, call to safe.execTransaction

[mmv08]

the discussion (thanks to Nico): https://gnosisinc.slack.com/archives/CQB729LLF/p1608741535300300