sstables: adjust static column name creation interface
sstables: adjust static column name creation interface

The column_name can be comprised of more than one element. This will
be the case for collections, that will embed part of the collection
data in the column name.

While doing this, we also take advantage of the infrastructure we have
added to composite. We are duplicating this logic unnecessarily, which
is prone to bugs. Those bugs are not just theoretical in this case: the
static prefix is not "followed by a null composite, i.e. three null bytes."
as the code implies.

The static prefix is created by appending one empty element for each element in
the clustering key, and will only be three null bytes in the case where the
clustering key has one element.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@cloudius-systems.com>

wildinto/scylla,asias/scylla,rluta/scylla,acbellini/scylla,gwicke/scylla,shaunstanislaus/scylla,bowlofstew/scylla,dwdm/scylla,stamhe/scylla,justintung/scylla,kangkot/scylla,guiquanz/scylla,senseb/scylla,gwicke/scylla,bowlofstew/scylla,respu/scylla,raphaelsc/scylla,justintung/scylla,dwdm/scylla,raphaelsc/scylla,rentongzhang/scylla,victorbriz/scylla,asias/scylla,rluta/scylla,scylladb/scylla,tempbottle/scylla,stamhe/scylla,capturePointer/scylla,scylladb/scylla,glommer/scylla,capturePointer/scylla,guiquanz/scylla,linearregression/scylla,asias/scylla,wildinto/scylla,duarten/scylla,kangkot/scylla,gwicke/scylla,eklitzke/scylla,acbellini/scylla,aruanruan/scylla,eklitzke/scylla,shaunstanislaus/scylla,glommer/scylla,duarten/scylla,tempbottle/scylla,duarten/scylla,avikivity/scylla,kjniemi/scylla,dwdm/scylla,eklitzke/scylla,raphaelsc/scylla,guiquanz/scylla,avikivity/scylla,respu/scylla,phonkee/scylla,senseb/scylla,capturePointer/scylla,aruanruan/scylla,senseb/scylla,aruanruan/scylla,rluta/scylla,glommer/scylla,kjniemi/scylla,victorbriz/scylla,acbellini/scylla,rentongzhang/scylla,wildinto/scylla,scylladb/scylla,stamhe/scylla,victorbriz/scylla,linearregression/scylla,avikivity/scylla,respu/scylla,kangkot/scylla,shaunstanislaus/scylla,justintung/scylla,tempbottle/scylla,kjniemi/scylla,scylladb/scylla,phonkee/scylla,bowlofstew/scylla,phonkee/scylla,rentongzhang/scylla,linearregression/scylla