Fill a data.table range with specific rows from read.fst

[MarcusKlik]

With this feature you can populate say row 1001:2000 in a 1e6 row data.table with a 1000 row read from fst.read. All this is done in memory. This feature is very useful for combining data from multiple (fst) sources into a single result table without having the overhead of copies. For example, when performing the merge sort algorithm on a set of data files, you need to

1. read first x rows from all files
2. sort the resulting table
3. write some rows to disk
4. read next x rows form file with smallest first chunk
5. sort resulting table
6. goto 3

This can be performed efficiently in R by using data.table's fast sorting and populating the result table in memory. With such an algorithm operating on a collection of fst files, we basically have a method of sorting arbitrary large fst files without running out of memory (and it can be done with multiple threads!).