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.

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