DAP: Investigate supporting multiple queued commands

[adamgreig]

CMSIS-DAP's DAP_Info command returns a 'maximum packet count', which reflects how many packets the probe can have on-the-go at once. As far as I can tell, this refers to an internal queue of incoming requests which are processed in-order and then results returned in-order in an outgoing queue. Currently the firmware returns '1' for this field, and we always send the result of a request before receiving the next one.

However, this limits USB throughput, especially in HID mode where each frame will either be a new command or a response, but not both. If we could implement a short queue it might help improve throughput. The DAP_TransferAbort command might make more sense too.