[cluster_test] Stop validators and measure throughput
[cluster_test] Stop validators and measure throughput

This diff is a first mix of experiments and throughput evaluation
It kills N validators 1 by 1 and measures how this affect throughput and latency

It prints out result in TSV format at the end, so it can be copy-pasted into excel to plot graph

Example:

```
$] ./cluster-test --workplace cluster-test --stop-experiment --max-stopped 30
Oct 04 01:01:36.857 INFO Discovered 100 peers
Oct 04 01:01:36.862 INFO Minting accounts
Oct 04 01:01:43.365 INFO Mint is done
Oct 04 01:03:14.467 INFO Tps: 688, latency: 901 ms
Oct 04 01:03:15.532 INFO Stopped 1 validators: 85ef1c9c
Oct 04 01:03:47.293 INFO Minting accounts
Oct 04 01:03:47.293 INFO Mint is done
Oct 04 01:05:18.378 INFO Tps: 572, latency: 1160 ms
Oct 04 01:05:19.768 INFO Stopped 2 validators: 85ef1c9c,53579dca
Oct 04 01:05:51.643 INFO Minting accounts
Oct 04 01:05:51.643 INFO Mint is done
Oct 04 01:07:22.723 INFO Tps: 594, latency: 1086 ms
Oct 04 01:07:23.939 INFO Stopped 3 validators: 85ef1c9c,53579dca,49c5651f

<...>
Results in csv format:
0	688	901
1	572	1160
2	594	1086
3	540	1209

```

Closes: #1186
Approved by: phoenix-antigravity

libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core