Common base for fsttable and fstplyr

[MarcusKlik]

These two interfaces to the underlying fst file should use the same mechanism for keeping the minimum amount of data in memory. Therefore, the fst package should define methods to accommodate both, probably first as non-visible package methods.

[MarcusKlik]

That common interface is defined in the fst_proxy class and accompanying methods. In time, this intermediate interface could serve as the base for both fsttable and fstplyr (and possible other interfaces)