cause amqp and kafka inputs to not consume messages when message processing is disabled

those types of inputs that support back pressure should listen on Lifecycle changes and refrain from reading more messages from their respective sources, if possible.
the amqp input now also does not reconnect if graylog2 disconnected explicitly. this could lead to consuming messages even though we tried to shut down.

for the purpose of propagating the lifecycle changes there is now an AsyncEventBus available. currently only Lifecycle instances are used on it.

fixes #320