Limit number of socket opened at once

[fmorency]

Running many-fuzzy with -n 1000 will open 1000 connections, i.e., files. We should limit the number of connections.

Steps to reproduce

# Raise your open file limits
$ ulimit -n 100000

# Comment ledger-related stuff in the `run.sh` script from `many-framework'

# Start the `many-framework` `run.sh` script
$ ./script/run.sh

# Wait for it to initialize properly ...

# Get the `many-abci` PID.
$ ps -C many-abci -o pid=
[SOME_PID]

# Start a terminal and watch the `many-abci` open file list
$ watch -n 0.5 'ls -l /proc/[SOME_PID]/fd'

# Start another terminal and watch the `many-abci` open file count
$ watch -n 0.5 'ls -l /proc/[SOME_PID]/fd | wc -l'

# Send 1000 requests to the `many-abci`
$ many-fuzzy -v --nonce fuzz --async --pem=id1.pem -n 1000 http://127.0.0.1:8011 'kvstore.put' "{0: h'0%(uint in 1..=9)', 1: h'01'}"

Notice that the file count will raise over 2000. We are opening 1000 socket (files) in many-fuzzy and the same amount in many-abci, which is another issue.