Dyntftpd

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* No entry.

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* In HTTP handler, the cache filename contains client's source port and human
readable datetime, making debugging easier.
* We no longer create a thread for each request.
* API break: move load_file and unload_file from TFTP handlers to Session.
* Use CleverHandler instead of FileSystemHandler by default.
* Free resources if client disconnect before completing a transfert.

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* Add unload_file callback in TFTPUDPHandler, called when the transfer of a
file is over and successful.
* Remove cache files for successful transfers in HTTPHandler.

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* Create HTTPHandler.
* Create CleverHandler to dispatch to HTTPHandler or FileSystemHandler
depending on the requested file.

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* Use ThreadingMixin to serve multiple requests at the same time.
* Accept option blksize.

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* Accept, and ignore extra request options.