switch to TMVars for thread safety when using the async extension

switch to TMVars for thread safety when using the async extension

TVars were not updated atomically, which was ok when each thread got its
own External that was the only thing using these TVars. But, with the
async extension, several External instances can share the same var, so
it needs to be a TMVar to avoid read/write conflicts.

In particular, this makes PREPARE only be sent once.