CRssManager: add locking to protect maps
CSettings/CRssManager: no need to call Clear() in destructor

Calling Clear() in CSettings destructor results in a call to
CRssManager::Clear() but depending on the destruction order of static variables
the CRssManager instance may have already been destroyed which can result in
an access violation exception.
Merge pull request #2420 from Montellese/rssmanager_locks

fix possible access violation in CRssManager::Clear() during shutdown