With this new layout, sparsehash will be used over default HashMap,
sparsehash is more memory efficient but it is also slower.
So in a nutshell:
- HashMap uses ~2x more memory then sparse_hash_map
- HashMap ~2-2.5x faster then sparse_hash_map
(tested on lots of input, and the most close to production was
dictionary with 600KK hashes and UInt16 as value)
TODO:
- fix allocated memory calculation
- getBufferSizeInBytes/getBufferSizeInCells interface
- benchmarks
v0: replace HashMap with google::sparse_hash_map
v2: use google::sparse_hash_map only when <sparse> isset to true
v3: replace attributes with different layout
v4: use ch hash over std::hash
