Handle ulimit

[Fizzadar]

When deploying to lots of hosts, using lots of files, pyinfra often hits the file open limit. This is because a new file object is opened per-file, per-host (so uploading the same file in parallel work). Use Python's resource module to guestimate good values for config.PARALLEL.

During an operation, possible open files/host:

1. Main one for commands
2. Second SFTP connection
3. Local file for SFTP
4. Two per nested local Popen commands (callbacks)
5. Nested fact commands (callbacks) uses main SSH connection

Max files / host should be 3 - which is SSH + Popen. SFTP uses main SSH connection and file handle is closed immediately after - so will never clash with Popen/etc.

[Fizzadar]

Min open files and host counts (using local.shell within a callback` and a file upload via SFTP):

25 files / 5 hosts
28 files / 6 hosts
31 files / 7 hosts

IE: hosts * 3 + 10 seems to be the magic formula for nofile ulimit.

[Fizzadar]

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