FPGA should acknowledge received event requests

[okahilak]

• The acknowledgement message could contain the time of receiving the message, which can be used to infer the minimal time that events needs to be sent in advance, e.g., in the real-time pipeline.
• One possible implementation would add an internal FIFO for each event type-channel combination for the acknowledgement messages, and the contents of those FIFOs would be multiplexed to a single target-to-host FIFO, similar to what is currently done with event feedback messages.