Add some code to Assert that when we release pin on a buffer, we are

Add some code to Assert that when we release pin on a buffer, we are
not holding the buffer's cntx_lock or io_in_progress_lock.  A recent
report from Litao Wu makes me wonder whether it is ever possible for
us to drop a buffer and forget to release its cntx_lock.  The Assert
does not fire in the regression tests, but that proves little ...