Replace ExecutorService with BucketedExecutor.

[bturner]

Using the ScheduledExecutorService is better than performing the pushes directly in the hook method, but it still has limitations:

• Busy repositories can result in parallel pushes to the mirror (#23)
• The ScheduledExecutorService being used is a shared resources, which can result in other callers being starved out

A better approach is to use the BucketedExecutor. It offers several improvements that are tailor-made for the work being done here:

• Buckets can be used to control what can be done in parallel
  • Tasks with the same key are grouped, and at most a single node in a Data Center cluster can run them
  • Tasks with different keys can be run in parallel
• Work can be shared among cluster nodes, in Data Center installations
  • Tasks can be scheduled from one cluster node and run on another
• Locking is inherent in the BucketedExecutor's design, which means the hook doesn't need to do any locking of its own
  • This means there aren't any blocked threads.

This fixes issue #23 by ensuring at most a single push happens to any given mirror, and by allowing the overall concurrency, even in Data Center installations, to be controlled. (#34)

While I was making changes, because the minimum supported version for the plugin is already Bitbucket Server 5.5, I switched the code over to the new PostRepositoryHook SPI. This could be extended to allow the hook to respond to a wider array of events, so mirroring would be triggered after pull request merges, branch or tag creation, etc. However, for the sake of consistency, the current code still only triggers pushes after other pushes. (Related to #45)

Other things:

• Added the ability to configure a timeout (#39)
• Added the ability to configure the number of retries, and the number of BucketedExecutor threads per cluster node
• Added the ability to pass -Dbitbucket.test.version=5.11.1 (or any other version) to test against a version of Bitbucket Server other than the one being compiled against
• Marked the plugin Data Center-compatible
• Simplified wiring for DefaultPasswordEncryptor and removed the init method from the PasswordEncryptor interface

[bturner]

(Note: I'm the principal developer for Bitbucket Server. I'm submitting this in the hopes it will be useful, even if for nothing other than to be a basis for your own changes.)

This change is large, so you may or may not want to accept it. I've done some basic testing, of course, and everything works. More testing is almost certainly necessary. If you feel like the changes are worthwhile, though, I hope you can accept them.

[adrianluisgonzalez]

Thank you so much @bturner for all these changes! Sorry it took me 9 months...