Commitlog: do not eagerly create first segment on init
Commitlog: do not eagerly create first segment on init

Deferring makes it easier to separate old segments from new, which in turn
helps replay logic.

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