For ENG-4395, ENG-4398, ENG-4397, add tests to elastic hashinator

Test that C++ matches Java. This is done by paramaterizing the existing hashinator test suite to work with both.

I also updated STX btree to the latest version removing the memory pooling code we were using. It was making testing the hashinator difficult and is of dubious value. Better to LD preload a jemalloc.