deadlock in IGMP streaming server when switching live TV.
Previously cComponentIGMP::Accept did all the work including the channel
switch with the new cConnectionIGMP waiting for MainThreadHook. But as the
new connection is stored *after* Accept, MainThreadHook didn't see it and
so wasn't able to switch. The streamdev main thread waited forever.
Moved the main work into cComponentIGMP::Welcome.