Refactoring of the XMLRPC subsystem in order to handle proper setup/cleanup of the XWiki context

This refactoring centralizes the setup of the XWiki context that is made available to all XMLRPC methods and allows for its proper setup/cleanup.
The XMLRPC servlet is configured so that the following pattern is used for calling an XMLRPC method:

try {
Setup of the XWiki context.

Actual XMLRPC method invocation through Apache XMLRPC.
}
catch(Exception e) {
Exception handling.
}
finally {
XWiki context cleanup.
}

This code is in the XWikiReflectiveXmlRpcHandler#execute method.