BUG: Non-unique indexing via ``loc`` and friends fixed (GH3659_)

BUG: deal with non_monotonic indices

CLN: convert slice_locs arrays to sliced ranges if possible
CLN: did not need to convert to index array/slicer as the only

     time this happens is when a boolean array comes back from get_loc,
     means the index is non_monotonic, which is an exception in any event
Merge pull request #3661 from jreback/loc_non_unique

BUG: Non-unique indexing via loc and friends fixed when slicing (GH3659_)