Workaround flaw in ScheduledThreadPoolExecutor.execute()

[keith-turner]

ScheduledThreadPoolExecutor.execute() will silently eat uncaught exceptions. This change makes calls to that method fail in order to prevents its use in Accumulo.

This impl code in java returns a future that is dropped on the floor. That future object will catch any uncaught exceptions. The javadoc for the method states Instead, the {@code Throwable} thrown by such a task can be obtained via {@link Future#get}. however that future is inaccessible so not sure how it expects that do be done.

[keith-turner]

Created this method as a draft because I need to determine if this execute method is actually called in the 2.1 code. I ran mvn verify -Psunny and that passed. Need to exclude calls to ThreadPoolExecutor.execute when searching because that is ok, trying to figure out how to do that.

If I can not determine if this is called in the 2.1 code, then may change this to warn in 2.1 and then fail in 4.0.

[keith-turner]

This was found while looking into apache/accumulo-classloaders#50

[keith-turner]

In 714062e worked around the flaw in the Java code instead of trying to avoid it. Also added a test to ensure the execute(Runnable) methods on threadpools do push uncaught exceptions to the handler. This new test would fail w/o the workaround changes. Taking this out of draf w/ the change in behavior. Did find some code in ConditionalWriter that is calling execute() on a scheduled executor, but not sure if its all.

[dlmarion]

LGTM