Fix threshold retrieve in tx-service mode

[moisses89]

Description

When the safe-cli is running on ts-service mode the threshold was wrongly updated to the new value during a change_threshold command call.
The issue was that the function prepare_and_execute_transaction is overwritten by the SafeTxOperator class with the difference that instead execute the transaction in blockchain, the overwritten function POST the transaction to the Safe Transaction Service without execute it.

Proposal solution

Update from blockchain the threshold value.

Related issue

Closes #323

[coveralls]

coverage: 87.036% (-0.1%) from 87.16%
when pulling efa6f28 on fix_reload_threeshold
into cc5b9f1 on main.

[coveralls]

coverage: 87.011% (-0.1%) from 87.136%
when pulling 70133fb on fix_reload_threeshold
into 29be0a2 on main.

[coveralls]

coverage: 86.934% (-0.1%) from 87.033%
when pulling 6ba5670 on fix_reload_threeshold
into bebbba2 on main.