Loading multiple versions of Tor confuses preloading

[robgjansen]

We store one single global function table for ALL Tor versions, but really we want a new function table for each individual Tor version.

The problem is inside of shadowtorpreload_init. This function gets called once for every different Tor version, but every time that happens it wipes out the old function table and the old library handle that is stored in per thread state. This problem will affect any thread that is configured to run nodes with different Tor versions.

Thanks for finding this @swojo!