Should clean up the stale StepRequest

[testforstephen]

For the sample code, when step over the line 5, actually it's broken by breakpoint at line 9. The pending StepRequest will never be clean up by the debuggee JVM. We should handle this kind of case to clean up the stale step requests.

[testforstephen]

A pending step need be cancelled if a breakpoint suspends execution during the step.

[testforstephen]

fixed at PR microsoft/java-debug#106