REST: upgrade Restlet from 2.0 to 2.1

Had to fix some minor compilation errors in `librairies/rest`. Works OK.

But, RssReaderTest in `libraries/rest-client` now behave badly and takes age to complete (recoverable error / refresh cycles).
Note that I did not change any code in the `librairies/rest-[common|client|server]` modules and that `librairies/rest` is a different thing.

Could anybody with more Restlet practice than me take a look please?

Cheers

/Paul