Almost* lock-free implementation of hash-map, free of limitations other lock-free hash maps impose.

• Allows rehashing without blocking lookups or insertions across the entire range of keys
• Allows any type of key, isn't limited to atomically CAS-able
• Good performance in concurrent scenarios
• Single-thread lookups are as fast as std::unordered_map
• Infinitely extensible
• Erasures free memory, no tombstones
• Does not rely on any reserved invalid key value

• As the actual size of the map grows beyond the expected capacity, insertion, lookup and erase time complexity degrades to O(log log n)
• Currently, there are opportunities for the map to be less memory-hungry if lock-free atomic shared pointers are implemented, albeit it's still ok without them
• Relies on hazard pointers for safe key deletion - latency is bad in the worst case
• *Rehashing requires locking and has the same latency problem (this could be solved with each Put doing a smaller part of the rehashing)
• Extremely reliant on a good hash-function
• Requires a seeded hash-function in order to be reliable in worst-case scenarios
• Currently does not have iterators
• Does not shrink.

See paper.md for details.