[NNTP] Can fail to disconnect when the command was cancelled
[NNTP] Can fail to disconnect when the command was cancelled

There was used already cancelled cancellable for the service
disconnect, which could succeed, but as the cancellable was
cancelled the overall operation failed and the NNTP store was
left in the 'connected' state, even with no nntp_stream set.

matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server