Storing intermediate results in registers is not worker threadsafe.

[guyboertje]

The same instance of the plugin is used across worker threads. The registers hash is therefore shared and not isolated by thread.

Need to introduce a concurrent hash keyed by thread. PR to follow.

[numbnut]

It would be nice to know, whether this is still an issue. In the commit history it looks like this issue is already covered. If yes, this ticket could be closed. Thank you!