[Lazarus] FCL-Web updates/extensions

[Michael Van Canneyt]

Hello,

Thanks to the efforts of Ondrej Pokorny and some sponsored work by the
FPC/Lazarus foundation, FCL-Web has received some updates:

- Improvements in fcl-net, basically a Select() call has been added to the
   socket stream, on all platforms that support this.

- Possibility to keep connections open in the HTTP Server (keep-alive).

- A HTTP client connection pool, for parallel downloading.

- HTTP Client Downloads can now be canceled.

- New threading model for the FPC HTTP server:
   - No threading
   - One thread per request/connection
   - Thread pool, distributing requests over available threads.

- The thread pool functionality is in a separate unit, available for other things as
   well...

- Interceptors in the HTTP router:
   In the HTTP router you can now register interceptors.
   These will be run before/after the request is dispatched to the correct handler.

   Interceptors can cancel further handling of a request:
   This allows you to implement e.g. authentication, logging etc. in an
   orthogonal manner to normal request handling.

- The FPC HTTP server can now handle Connection: upgrade using registered
   upgrade handlers. This can be used for websocket (see below) and later
   HTTP/2 handling.

- FPC has now native support for the websocket protocol: both client and
   server are supported. Native chat client/server demos are included,
   and a pas2js client for use in the browser is also available.
   (SSL support is there but still needs testing)

- The websocket protocol support can be attached to the HTTP server. That
   means you can initiate a websocket connection on the same port as the HTTP
   server is listening; the HTTP server will hand off the connection to the
   websocket server using the upgrade mechanism. A server demo is included as well.

Attached a small screenshot of the websocket-based chat program in pas2js.

Enjoy,

Michael.
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