parallel:::.slaveRSOCK() -> parallel:::.workRSOCK() name change in R (>= 4.1.0)

[schklom]

As mentioned in HenrikBengtsson/future#393, an update to parallel renamed parallel:::.slaveRSOCK() into parallel:::.workRSOCK().

partools needs to call the correct function depending on the R version. It is currently broken on the new R versions.