MINOR: stktable: add sc[0-2]_key fetches

MINOR: stktable: add sc[0-2]_key fetches

As discussed in GH #1750, we were lacking a sample fetch to be able to
retrieve the key from the currently tracked counter entry. To do so,
sc_key fetch can now be used. It returns a sample with the correct type
(table key type) corresponding to the tracked counter entry (from previous
track-sc rules).

If no entry is currently tracked, it returns nothing.

It can be used using the standard form "sc_key(<sc_number>)" or the legacy
form: "sc0_key", "sc1_key", "sc2_key"

Documentation was updated.