Bubble Throwables when executing Runnables

[jeremysears]

This change updates calls to ExecutorService.submit (mostly deamons), to
call ExecutorService.execute instead. The submit variant returns a
Future which will contain any errors that are thrown from the Runnable.
However, nothing was checking these throwables for errors. If an error
happens, for example an OutOfMemoryError or RuntimeException thrown from
a custom AWSCredentialsProvider, the deamon thread will die without ever
logging any error information. Changing this to use execute allows the
Thread's UncaughtExceptionHandler or the default
UncaughtExceptionHandler to properly handle the failure. At a minimum
this allows the error to be logged. Some clients may wish to respond to
an OutOfMemoryError by taking a more severe action, such as restarting
the service. Given that failure of these deamon threads will likely
wedge the KPL, some retry logic or a hard shutdown should probably be
implemented in a subsequent commit.

(partially)
fixes #111

[jeremysears]

I've updated the PR, so that it doesn't contain superfluous white-space changes.

[pfifer]

Please confirm that we can use, modify, copy, and redistribute this contribution. Thanks.

[jeremysears]

I confirm that you may use, modify, copy, and redistribute this change.