Implemented synchronized PriorityQueue.

[jdantonio]

The original priority queue implementation was added as an internal implementation detail of TimerSet. It was not thread-safe because TimerSet handled the synchronization. I recently began experimenting with a Rails ActiveJob adapter built on concurrent-ruby. I began thinking about adding prioritization to the adapter and realized that I might be able to use our priority queue, so I took a stab at creating a thread-safe wrapper around our current implementation.

This is a low-priority item since it doesn't look like my Rails PR is going to be merged. But this update may still be a nice addition to our gem.

This PR does a few things:

• Renames the original priority queue implementation to better reflect that it is not thread-safe.
• Removes the unnecessary method aliases from the non-thread-safe priority queue implementations.
• Implements a synchronized Concurrent::PriorityQueue which leverages Concurrent::Synchronization::Object.
• Implements the original method aliases on Concurrent::PriorityQueue.

[jdantonio]

I'm going to close this PR until we have an actual use case. I'm going to cherry-pick commit 677ab3e onto master. The classes need renamed to better reflect their not-thread-safe status.

[jdantonio]

Superseded by PR #399