splinter_test --seq-perf key generation is not coherent between insertion and lookup phases

[ajhconway]

It uses key generation that is based on thread_number and insert_number, but then it uses a global counter with batching to determine the insert_number, so that which thread_number, insert_number pairs are inserted is non-deterministic. As a result, the lookup phase always fails unless a single thread is used for both insertion and lookups.