MEDIUM: stktable: leverage smp_fetch_* helpers from sample conv

In this patch we try to prevent code duplication: some fetches and sample
converters do the exact same thing, except that the converter takes the
argument as input data. Until now, both the converter and the fetch
had their own implementation (copy pasted), with the fetch specific or
converter specific lookup part.

Thanks to previous commits, we now have generic sample fetch helpers
that take the stkctr as argument, so let's leverage them directly
from the converter functions when available. This allows to remove
a lot of code duplication and should make code maintenance easier in the
future.