Implementation issue following the discussion in #81.

Specifically the RPC should block until at least one event is accepted into the queue. The RPC should not block until all events in the batch have been accepted into the queue.

The shipper queue's publish interface already blocks when the queue is full based on the underlying Beats' memory and disk queue implementations:

1. Beats' memory queue publish implementation: https://github.com/elastic/beats/blob/e6db9a53f2fff60b350153b03407bc38b21bf0b1/libbeat/publisher/queue/memqueue/produce.go#L128-L136
2. Beats' disk queue publish implementation: https://github.com/elastic/beats/blob/e6db9a53f2fff60b350153b03407bc38b21bf0b1/libbeat/publisher/queue/diskqueue/producer.go#L52-L62

One missing piece with the existing Publish method is that it does not accept a context.Context as input. This means that when a PublishEvents RPC call is made with a timeout then that timeout will be ignored if the RPC is blocked in the queue Publish method. We will need to modify the queue interface to accept a context.Context and propagate the RPC context to it in the PublishEvents method.