MINOR: stick-table: change the API of the function used to calculate the shard

MINOR: stick-table: change the API of the function used to calculate the shard

The function used to calculate the shard number currently requires a
stktable_key on input for this. Unfortunately, it happens that peers
currently miss this calculation and they do not provide stktable_key
at all, instead they're open-coding all the low-level stick-table work
(hence why it's missing). Thus we'll need to be able to calculate the
shard number in keys coming from peers as well but the current API does
not make it possible.

This commit addresses this by inverting the order where the length and
the shard number are used. Now the low-level function is independent on
stksess and stktable_key, it takes a table, pointer and length and does
all the job. The upper function takes care of the type and key to get
the its length, and is for use only from stick-table code.

This doesn't change anything except that the low-level one will be usable
from outside (hence why it's exported now).