SyncArbiters can close with active messages in queue

[Firstyear]

Expected Behavior

SyncArbiters can close where there are active messages in their queues. Normally this behaviour is fine, but for example, a syncarbiter acting as a logging thread, this can cause information to be lost.

Current Behavior

In the context, it should be possible to specify the stop conditions, such as "just stop" or "stop once drained" for example. Normally stop order comes from the msg.connected(), which means that once there are no more connections, it should be possible to drain the queue, and exit the arbiter cleanly.

• Actix Version: 0.9, but examining the code, this would occur in 1.0.0

I think this could be achieved simply by adding a "is_empty()" to msgs, which returns "state.num_messages == 0", then in sync.rs, the stop condition would be "if connected || !is_empty()"

I can supply a patch if needed.