Flush the monitor's read region on stop if and only if it has already been initialized

[aliddell]

In acquisitions where we call

runtime.start()
runtime.stop()

without inspecting the data at any point in between, the read-region flush on acquire_stop initializes the monitor. Calling this multiple times fills the monitor without ever flushing it, causing the runtime to hang. This fixes that problem.

[aliddell]

Confirm that the test in the new file repeat-start-no-monitor.cpp will hang without the change made in acquire.c.

[aliddell]

Closed in favor of #44