`between` on POSIXct fallback to slow character field processing

```r
require(data.table)
x = as.POSIXct("2016-09-18 07:00:00") + 0:10*60*15
dn = as.POSIXct('2016-09-18 08:00:00')
up = as.POSIXct('2016-09-18 09:00:00')
between(x, dn, up, incbounds=FALSE, verbose=TRUE)
#between on character field, fallback to slow R routine
```