Opentelemetry-java: Replace usages of java.util.Timer with a ScheduledExecutor in the BatchSpanProcessor

[jkwatson]

The Timer has known issues with clock changes. We should use ScheduledExecutors as they are the more modern construct for doing timed executions.

[anuraaga]

Is this a dupe of #729?

[jkwatson]

It's smaller in scope. The recent changes to make export async introduced a Timer. It wasn't there before. If #729 were done, I would assume this one could be closed.

[malafeev]

I can do it, please assign to me to proceed.

[iNikem]

There are already 2 PRs addressing this issue :)