Allow for queueing messages

When agent is running new messages cannot be sent. One has to wait for the round to complete to send next message. It would be useful if queueing / steering was allowed.

No file or test is named. Start by locating the agent message-handling entry point and tracing what blocks new messages during a round; define how queued and steering messages should behave, then add coverage showing messages can be submitted while a round runs and are handled as intended.

Written by the indexing model from the issue text.