Do not reference RequestGroups longer than necessary

There is an initial vector of SharedHandle<RequestGroup> to seed the
DownloadEngine. This vector was however kept alive via main.cc ->
MultiUrlRequestInfo up until the program exits, hence effetively leaking
all initial RequestGroups and associated object instances.

Hence, as a matter of dirty-workaround, drop the contents of that initial
vector as soon as it is not required any longer.
Merge branch 'fix-initial-rq-leak' of https://github.com/nmaier/aria2 into nmaier-fix-initial-rq-leak