DAL/P2p: Add `--max-opened-p2p-connections` option

Fixes #6140

Stacked on !16681 (merged)

What

Introduces a command line argument --max-opened-p2p-connections.

Why

To limit the number of file descriptors a DAL node can open.

How

A P2p_fd.fd_pool (introduced in !16681 (merged)) is used, which is created in the DAL node, and an element of this pool is taken at each connection opening, element which is released at the connection closing.

Manually testing the MR

Manual experiments are detailed in a comment in this merge request.

Checklist

• Document any change to the user interface, including configuration parameters (see node configuration)
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).