Simplify txbuilder and improve keybase signing

[NicolasMahe]

This PR remove the custom TxBuilder that we have in favour of using the default one directly.
The custom keybase signing has been re-implemented to add a cache on the plain private key of the user. This impressively improves the performance! The engine can sign 1,000 of txs in less than 1 sec (before only around 3 or 4 per sec)

Related to #1570

Next step is to investigate mempool issue by running the monitoring tool

[antho1404]

I don't really like the management of the cache for the private key.

• The delay might be unnecessary for now
• The management of the lastSign is not clear. We should support have a map to cache all of them or none of them.

[NicolasMahe]

I don't really like the management of the cache for the private key.

• The delay might be unnecessary for now
• The management of the lastSign is not clear. We should support have a map to cache all of them or none of them.

I remove the cache based on time in favour of a cache using a map to store all private keys used. The index of the map is the sha256 checksum of the account's name and password.

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Performance comparison with the e2e tests execution/many_executions_in_parallel with a block time of 1sec that creates 10 executions:
before: 13.31s
after: 2.10s

🍾 🎊

[antho1404]

Would be nice to have the actual performance improvement with some metrics like #1570 (comment) to really see the impact of it ;)

[NicolasMahe]

Here is the result before and after with the monitoring metrics:
before:

everything is stuck and i had to stop the process at 14:34

after:

no issue at all. ALL erc20 events are saved to influxdb!!
45,000 txs executed in 45min. and that's not even the maximum!

@antho1404 check again now we have the dashboard in place!