Switch to lockless data structures

[penberg]

The current code uses a mutex to essentially make commit() and rollback() and other operations atomic. Let's switch to lockless data structures like Hekaton.

[penberg]

AFAICT, Hekaton uses a Bw-Tree for its index. However, the "Building a Bw-Tree Takes More Than Just Buzz Words" paper by Wang et al (SIGMOD’18) claims that skip list, adaptive radix tree, and masstree performs better, so maybe let's look into those. For example, https://docs.rs/crossbeam-skiplist/latest/crossbeam_skiplist/ might be a good candidate.