Add tracing and stats to OrderedSafeExecutor for debugging slow tasks

Porting a change form the Twitter branch to improve stats and logging in OrderedSafeExecutor
These changes have been helpful for us in debugging latency issues in Bookkeeper server/client
Summary of changes is

• add a config option for op stats
• add stats for task execution time, task pending time
• add a config option for logging a warning when an op takes longer than x micros
• add toString implementations for submitted tasks so make it easier to track down slow ops
• start using Builder for OrderedSafeExecutor
• add a very simple test to make sure that the slow op logging path is exercised

Most of this came from Sijie originally, with some changes from me