TCP: fixes backlog limit handling, and allows ephemeral TCP connections

[d-a-v]

When a sketch does not check server::available() often enough, short connections are now received and stored in background (backlog limit number of connections at most) and can be retrieved and used by subsequent client = server::available(). Those clients will be status() == CLOSED, connected() == false but available() > 0.

@kirillandy Could you check this ? I get what's expected by #2569 with these changes.

@JAndrassy This is still not what you are waiting for but it would be nice if you checked nothing has changed on the arduino side (WiFiClient::operator bool() has changed).

fixes #2569

edit:

fixes #7103

[kirillandy]

Hi @d-a-v ! Currently trying to check your changes out, but I'm not too certain about my actions.
So I installed the current ESP Arduino library version using Git (as the docs instruct). After a bit of research I then used "git merge https://github.com/d-a-v/Arduino.git FixBacklogHonorEphemeral" to apply your changes on what I have locally stored on my PC.

Is that all I need to do before I compile my code to test out on my ESPs?
(Also, if my actions were correct, was there an easier way to do what I did? :D)

[kirillandy]

UPDATE:
So, if everything I did to get hold of your commit was correct (still need some friendly guidance on that, since this is my first time properly using Git and its commands :)), then everything works like a charm! I repeated my previous experiment by sending a few bytes when the server was not available-looping and the server was able to get the client and read the bytes 👍
I also tried sending bytes in 3 different connections one after the other to check if they'd all be in the queue and the server managed to .available() all 3 and read every message.
Thanks a lot for your work, @d-a-v!

[devyte]

LGTM

[Jason2866]

Fix #7103