Request Event Driven (non-blocking) WiFi socket API

The request is for a method call which returns true iff a call to WiFiClient.write(buffer) will NOT block.
as in

```
if (client.canWriteBufferWithoutBlocking()) {
client.write(buffer); // this will NOT block
}
```

The low level API has a callback framework, so this could be implemented by a boolean which is cleared on a call to client.write() and set by the callback that is executed when the buffer has been sent and ACKed.

(Also as another request, can client.flush() be a noop, so incoming data not lost, but kept until read or client stopped.)

Background to this Request ==========
(see https://github.com/esp8266/Arduino/issues/917)

ESP can only handle one outgoing TCP packet at a time. It needs to keep the last packet for retransmission until it is ACKed from the other side. For Windows clients the ACK delay is 0.2 secs.

While ESP is waiting for ACK, it cannot accept another client.write(buffer)
So currently "client.write() blocks until either data is sent and ACKed, or timeout occurs (currently hard-coded to 5 seconds). Also client.flush() doesn't "force" sending data after client.write(), because there is no output buffer which can be flushed. It does discard incoming data though."

If the client is connected then you could expect that after a maximum of 0.2 sec blocking, the next data buffer will be written. However if the connection is lost, (i.e. no return ACK) then the sketch will block for 5 sec.

In many cases both the 0.2 sec and 5 sec blocking is too long.

In my particular case of a UART to WiFi bridge sketch, 0.2 sec at 115200 baud is about 2K of incoming serial data that is being ignored / lost while the sketch is blocked in client.write();
##

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