fread should read columns of Date/POSIXct types directly

[arunsrinivasan]

[statquant]

May be use fasttime to read POSIXct/Date quickly like here

[MichaelChirico]

@statquant thus relating this to #587

[MichaelChirico]

Closed by #2545 ?

[jangorecki]

@MichaelChirico agree we can close this one, @arunsrinivasan please re-open if needed, ideally providing a use case, so it can be added to unit tests.