Netty: Make sure channel closing never happens on i/o thread

Similar to NettyTransport.doStop() all actions which disconnect
from a node (and thus call awaitUnterruptibly) should not be executed
on the I/O thread.

This patch ensures that all disconnects happen in the generic threadpool, trying to avoid unnecessary `disconnectFromNode` calls.

Also added a missing return statement in case the component was not yet
started when catching an exception on the netty layer.

Closes #7726