Use refcount(9) to manage references. Partially tear down the sim in
cam_sim_free so that all further calls to sim_action and sim_poll fail.
Set the mtx = NULL so no further locking is done for this sim. Document
the contract with the sim better, especially for pending requests that
come in prior to this call. cam_sim_free() is no longer blocking, but as
far as the client SIM is concerned, "sim" has been disposed of and the
SIM no longer owns "sim" memory. This also eliminates the
cam_sim_free_mtx for those SIMs that have to registered lock.
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