Float64Index is very slow in some condition.
The following code is very slow:

```
import pandas as pd
import numpy as np

dt = 4.8000000418824129e-08
data = np.random.rand(1000000, 4)
df = pd.DataFrame(data, columns=list("ABCD"))
df.index *= dt
print(df.loc[0:0.001].shape)
```

after debug it, I found `Float64Engine.get_loc()` is slow. Here is a demo:

```
import pandas as pd
import numpy as np

a = np.arange(1000000)
ind1 = pd.Float64Index(a * 4.8e-08)
ind2 = pd.Float64Index(a * 4.8000000418824129e-08)

%time ind1._engine.get_loc(0)
%time ind2._engine.get_loc(0)
```

outputs:

```
Wall time: 295 ms
Wall time: 9.9 s
```
