Many thanks to Peter and Darian for clearing up my misunderstanding of
how vkResetCommandPool works. The manager creates command buffers from
the command pool on an as-needed basis (when the queue of available
buffers is empty), and keeps track of those buffers in a queue. When the
pool is reset, the queues (one each for primary and secondary command
buffers) are reset such that the tracked buffers are available again.