Fixed lockup & segfault bugs in native code; Cleaned up perf testing module

Fixed incorrect sizing of ed25519 signature buffers. This caused ed25519_sign to
write past the end of the buffer triggering random segfaults. A lock up condition
occuring when the NIF was unloaded has been fixed by simplifying the destruction
process for the shared worker thread queue.

The brine_perf module has also received a few tweaks and clean ups.