Contention and excessive overhead creating digest instances

[headius]

The following script, from @bbrowning, shows that we have a large object churn and classloader contention when creating digest instances, due to the logic in findImplEngine running for every new instance.

https://gist.github.com/bbrowning/24cfdba60ce1cdc887cc

I've got a patch on the way. Unsure how we can test this, though, since we'd have to confirm it runs in a certain amount of time to confirm the contention is gone.

[kares]

oh najs, btw. the code as it was should have been already slightly faster / improved than 0.9.5 ...