Rely on the kernel for port allocation

[lexnv]

The testing infrastructure requires that a substrate binary is present for individual testing.
However, having a different polkadot binary running resulted in failing tests.

Root Cause

The function next_open_port() in subxt crate produced 3 port numbers by inspecting
a range of ports.
To validate that a given port is opened the TcpListener::bind(("0.0.0.0", 0)) bind call is executed.
However, the address "0.0.0.0" is not representative of the localhost, but of a non-routable unknown address.

This observation is validated using the polkadot binary which starts by default on 127.0.0.1:9944. Any TcpListener::bind(("0.0.0.0", 0)) will result in success even with the binary started.

Port Allocation

While at it, remove the scanning logic and rely entirely on the Kernel to provide 3 available ports.
The port allocation is performed by the inet_csk_get_port() function.
When providing a zero port, asking for allocation, the code path goes to inet_csk_find_open_port().
A port scanning is performed that uses a spinlock for concurrent access with the addition of checking for bind conflicts.

The kernel should provide 3 different ports if they are available.

Testing Done

Started 100 threads, each requesting for 100 new ports. Multiple runs of the binary presented no duplicates.
However, when requesting more ports than available (ie from 1000 threads), the ports are reutilized and values eventually wrap from net.inet.ip.portrange.last to net.inet.ip.portrange.first.

Closes #497.

[jsdw]

Excellent! Simpler and more reliable; the ideal combo :)

[niklasad1]

nice 👍