Implement inverse square in codel_impl.h without table lookup

[netoptimizer]

The traffic-pacing-edt/codel_impl.h code implements a table lookup for the inverse square root.

• as mentioned in PR #60

As Kathleen Nichols ( @kmnichols ) point out there's no need for a lookup table, as inverse square root isn't that expensive to calulate (only requires multiplies).

• http://pollere.net/CoDelnotes.html

I welcome someone to implement this.

[sachintiptur]

I can take up this.

Ref. calculation: https://en.wikipedia.org/wiki/Fast_inverse_square_root#Accuracy