A client making quick consecutive requests with keep-alive, such as `ab`
with `-k`, can cause the connection to become hung.
This happens because of an optimization in `http_poll` function. When a
connection state becomes `DONE`, `httpd_poll` recycles the connection
and immediately calls `poll_recv_request`. However, it doesn't handle
this resulting in the connection state becoming `DONE` again. If this
occurs, the state stays in `DONE`, and the further calls to `httpd_poll`
ignore the connection.
Fix this by calling `poll_recv_request` in a loop until the state is no
longer `DONE`.
* Enable TCP_NODELAY optimization
It looks like `TCP_NODELAY` was disabled due to the bug fixed in the
previous commit. Enabling it substantially improves keep-alive
performance with `ab`:
Before:
```
Time per request: 0.272 [ms] (mean)
```
After:
```
Time per request: 0.033 [ms] (mean)
```
* Remove keep-alive optimization from `httpd_poll`
Benchmarking with `ab` shows that bypassing `select` for keep-alive
connections in the `DONE` state doesn't significantly impact
performance. Since this optimization previously caused a bug, remove it.
