Fix null shutdown timeout in ExecutorServices

`ScheduledExecutorServiceBuilder` defines no default `shutdownTime` or
`shutdownUnit`. These are passed to `ExecutorServiceManager` which in
turn uses them verbatim on `ExecutorService#awaitTermination(long,
TimeUnit)`, potentially causing a `NullPointerException`.

Adding default values resolves this problem.
Merge pull request #327 from nicktelford/fix/null-shutdown-times

Fix null shutdown timeout in ExecutorServices