BUG/MEDIUM: port_range: Make the ring buffer lock-free.

BUG/MEDIUM: port_range: Make the ring buffer lock-free.

Port range uses a ring buffer, and unfortunately, when making haproxy
multithreaded, it's been overlooked, and the ring buffer is not thread-safe.
When specifying a source range, 2 or more threads could pick the same
port, and of course only one of them could use the port, the others would
always fail the connection.
To fix this, make it a lock-free ring buffer. This is easier than usual
because we know the ring buffer can never be full.

This should be backported to 1.8 and 1.9.