BufferPool is extremely slow under contention

[vkrasnov]

I am running a highly concurrent MySQL client and the Mutex contention in BufferPool leads to extreme slowdown.

I tried replacing the Mutex<Vec<_>> with crossbeam::queue::ArrayQueue and it led to a 4 fold performance improvement (in terms of QPS). I don't really see any downsides in using ArrayQueue even under low load, so if you would consider it I will be more than happy to open a pull request.

Thanks.

[blackbeam]

Thanks! Yeah, PRs are much appreciated.