Provide a runner-internal ExecutorService for maintenance tasks

There are multiple locations throughout the DirectRunner that require some work to be done at some later point (e.g. DirectMetrics.COUNTER_COMMITTER, EvaluationContext.watermarkCallbackExecutor). These should use a common ExecutorService to ensure that if necessary we can manage the shared JVM resources properly.

Imported from Jira [BEAM-723](https://issues.apache.org/jira/browse/BEAM-723). Original Jira may contain additional context.
Reported by: tgroh.

Start by tracing the DirectRunner locations named in the issue, especially DirectMetrics.COUNTER_COMMITTER and EvaluationContext.watermarkCallbackExecutor, and review the original Jira issue for context. Done means these maintenance tasks use a common runner-internal ExecutorService whose shared JVM resources can be managed consistently.

Written by the indexing model from the issue text.