jamiaccount: protect channelsHandlers_

src/call.cpp

src/client/conversation_interface.cpp

src/client/datatransfer.cpp

src/jamidht/conversation_channel_handler.cpp

src/jamidht/jamiaccount.cpp

src/jamidht/jamiaccount.h

src/jamidht/swarm/swarm_channel_handler.cpp

src/jamidht/sync_module.cpp

src/jamidht/transfer_channel_handler.cpp

src/manager.cpp