AVL Tree search is "slow"

[femtotrader]

Hello,

AVL tree integration in DataStructures.jl was discussed in #503 but it seems that this implementation is slow (especially for search operation).

See benchmark https://github.com/femtotrader/AVLTreeBenchmarks.jl/ for a comparison with @krynju implementation https://github.com/krynju/AVLTrees.jl and discussion p-casgrain/LimitOrderBook.jl#7 (LimitOrderBook.jl from @p-casgrain )

Best regards,
Femto