PERF: Float64Index to use Float hashtable backend
https://groups.google.com/forum/m/#!topic/pydata/zUxl7rOHVNY

Currently `Float64Index` uses the `Object` back-end for ease of implementation. 
See Here: https://github.com/pydata/pandas/blob/master/pandas/core/index.py#L1881
- change the backend to use `Float64Engine` - the cythob code needs fixin in order to make it work.
- needs vbench for perf indications
- needs pickle validation that it is backward compat (it should be w/o changes)
