RSS plugin getFeed and xwiki.getURLContent donc have timeouts and can lock a thread

Both the RSS Feed Api and the getURLContent xwiki api can lock a thread if connecting to a server that does not respond.
We need to implement a timeout (default 60 seconds).

For this the proposal is to use HttpClient for the Feed plugin and use it's timeout featur.
New apis are proposed for getURLContent including the timeout field.